Esta nieve en particular se queda pegada en la parte inferior de tu blog y se derrite pasado cierto tiempo. Cool ¿no?

Lo mejor es que no necesitas hacerle cambios a la plantilla.

Ve a Diseño -> Añadir un Gadget -> HTML/Javascript

Y pega el código (Lo sé es un poco largo u_u):

<script type="text/javascript">
//<![CDATA[
// Nieve en el blog
/** @license
DHTML Snowstorm! JavaScript-based Snow for web pages
--------------------------------------------------------
Version 1.42.20111120 (Previous rev: 1.41.20101113)
Copyright (c) 2007, Scott Schiller. All rights reserved.
Code provided under the BSD License:
http://schillmania.com/projects/snowstorm/license.txt
*/
var snowStorm=function(e,d){function g(a,d){isNaN(d)&&(d=0);return Math.random()*a+d}function o(){e.setTimeout(function(){a.start(true)},20);a.events.remove(i?d:e,"mousemove",o)}function r(){if(!a.excludeMobile||!s)a.freezeOnBlur?a.events.add(i?d:e,"mousemove",o):o();a.events.remove(e,"load",r)}this.flakesMax=128;this.flakesMaxActive=64;this.animationInterval=40;this.excludeMobile=true;this.flakeBottom=null;this.followMouse=true;this.snowColor="#ffffff";this.snowCharacter="&bull;";this.snowStick=true;
this.targetElement=null;this.useMeltEffect=true;this.usePositionFixed=this.useTwinkleEffect=false;this.freezeOnBlur=true;this.flakeRightOffset=this.flakeLeftOffset=0;this.flakeHeight=this.flakeWidth=8;this.vMaxX=5;this.vMaxY=4;this.zIndex=0;var a=this,y=this,i=navigator.userAgent.match(/msie/i),z=navigator.userAgent.match(/msie 6/i),A=navigator.appVersion.match(/windows 98/i),s=navigator.userAgent.match(/mobile/i),B=i&&d.compatMode==="BackCompat",t=s||B||z,h=null,k=null,j=null,m=null,u=null,v=null,
p=1,n=false,q;a:{try{d.createElement("div").style.opacity="0.5"}catch(C){q=false;break a}q=true}var w=false,x=d.createDocumentFragment();this.timers=[];this.flakes=[];this.active=this.disabled=false;this.meltFrameCount=20;this.meltFrames=[];this.events=function(){function a(b){var b=f.call(b),c=b.length;l?(b[1]="on"+b[1],c>3&&b.pop()):c===3&&b.push(false);return b}function d(a,c){var e=a.shift(),f=[b[c]];if(l)e[f](a[0],a[1]);else e[f].apply(e,a)}var l=!e.addEventListener&&e.attachEvent,f=Array.prototype.slice,
b={add:l?"attachEvent":"addEventListener",remove:l?"detachEvent":"removeEventListener"};return{add:function(){d(a(arguments),"add")},remove:function(){d(a(arguments),"remove")}}}();this.randomizeWind=function(){var c;c=g(a.vMaxX,0.2);u=parseInt(g(2),10)===1?c*-1:c;v=g(a.vMaxY,0.2);if(this.flakes)for(c=0;c<this.flakes.length;c++)this.flakes[c].active&&this.flakes[c].setVelocities()};this.scrollHandler=function(){var c;m=a.flakeBottom?0:parseInt(e.scrollY||d.documentElement.scrollTop||d.body.scrollTop,
10);isNaN(m)&&(m=0);if(!n&&!a.flakeBottom&&a.flakes)for(c=a.flakes.length;c--;)a.flakes[c].active===0&&a.flakes[c].stick()};this.resizeHandler=function(){e.innerWidth||e.innerHeight?(h=e.innerWidth-(!i?16:16)-a.flakeRightOffset,j=a.flakeBottom?a.flakeBottom:e.innerHeight):(h=(d.documentElement.clientWidth||d.body.clientWidth||d.body.scrollWidth)-(!i?8:0)-a.flakeRightOffset,j=a.flakeBottom?a.flakeBottom:d.documentElement.clientHeight||d.body.clientHeight||d.body.scrollHeight);k=parseInt(h/2,10)};this.resizeHandlerAlt=
function(){h=a.targetElement.offsetLeft+a.targetElement.offsetWidth-a.flakeRightOffset;j=a.flakeBottom?a.flakeBottom:a.targetElement.offsetTop+a.targetElement.offsetHeight;k=parseInt(h/2,10)};this.freeze=function(){var c;if(a.disabled)return false;else a.disabled=1;for(c=a.timers.length;c--;)clearInterval(a.timers[c])};this.resume=function(){if(a.disabled)a.disabled=0;else return false;a.timerInit()};this.toggleSnow=function(){a.flakes.length?(a.active=!a.active,a.active?(a.show(),a.resume()):(a.stop(),
a.freeze())):a.start()};this.stop=function(){var c;this.freeze();for(c=this.flakes.length;c--;)this.flakes[c].o.style.display="none";a.events.remove(e,"scroll",a.scrollHandler);a.events.remove(e,"resize",a.resizeHandler);a.freezeOnBlur&&(i?(a.events.remove(d,"focusout",a.freeze),a.events.remove(d,"focusin",a.resume)):(a.events.remove(e,"blur",a.freeze),a.events.remove(e,"focus",a.resume)))};this.show=function(){var a;for(a=this.flakes.length;a--;)this.flakes[a].o.style.display="block"};this.SnowFlake=
function(a,e,l,f){var b=this;this.type=e;this.x=l||parseInt(g(h-20),10);this.y=!isNaN(f)?f:-g(j)-12;this.vY=this.vX=null;this.vAmpTypes=[1,1.2,1.4,1.6,1.8];this.vAmp=this.vAmpTypes[this.type];this.melting=false;this.meltFrameCount=a.meltFrameCount;this.meltFrames=a.meltFrames;this.twinkleFrame=this.meltFrame=0;this.active=1;this.fontSize=10+this.type/5*10;this.o=d.createElement("div");this.o.innerHTML=a.snowCharacter;this.o.style.color=a.snowColor;this.o.style.position=n?"fixed":"absolute";this.o.style.width=
a.flakeWidth+"px";this.o.style.height=a.flakeHeight+"px";this.o.style.fontFamily="arial,verdana";this.o.style.overflow="hidden";this.o.style.fontWeight="normal";this.o.style.zIndex=a.zIndex;x.appendChild(this.o);this.refresh=function(){if(isNaN(b.x)||isNaN(b.y))return false;b.o.style.left=b.x+"px";b.o.style.top=b.y+"px"};this.stick=function(){t||a.targetElement!==d.documentElement&&a.targetElement!==d.body?b.o.style.top=j+m-a.flakeHeight+"px":a.flakeBottom?b.o.style.top=a.flakeBottom+"px":(b.o.style.display=
"none",b.o.style.top="auto",b.o.style.bottom="0px",b.o.style.position="fixed",b.o.style.display="block")};this.vCheck=function(){if(b.vX>=0&&b.vX<0.2)b.vX=0.2;else if(b.vX<0&&b.vX>-0.2)b.vX=-0.2;if(b.vY>=0&&b.vY<0.2)b.vY=0.2};this.move=function(){var d=b.vX*p;b.x+=d;b.y+=b.vY*b.vAmp;if(b.x>=h||h-b.x<a.flakeWidth)b.x=0;else if(d<0&&b.x-a.flakeLeftOffset<-a.flakeWidth)b.x=h-a.flakeWidth-1;b.refresh();if(j+m-b.y<a.flakeHeight)b.active=0,a.snowStick?b.stick():b.recycle();else{if(a.useMeltEffect&&b.active&&
b.type<3&&!b.melting&&Math.random()>0.998)b.melting=true,b.melt();if(a.useTwinkleEffect)if(b.twinkleFrame)b.twinkleFrame--,b.o.style.visibility=b.twinkleFrame&&b.twinkleFrame%2===0?"hidden":"visible";else if(Math.random()>0.9)b.twinkleFrame=parseInt(Math.random()*20,10)}};this.animate=function(){b.move()};this.setVelocities=function(){b.vX=u+g(a.vMaxX*0.12,0.1);b.vY=v+g(a.vMaxY*0.12,0.1)};this.setOpacity=function(a,b){if(!q)return false;a.style.opacity=b};this.melt=function(){!a.useMeltEffect||!b.melting?
b.recycle():b.meltFrame<b.meltFrameCount?(b.meltFrame++,b.setOpacity(b.o,b.meltFrames[b.meltFrame]),b.o.style.fontSize=b.fontSize-b.fontSize*(b.meltFrame/b.meltFrameCount)+"px",b.o.style.lineHeight=a.flakeHeight+2+a.flakeHeight*0.75*(b.meltFrame/b.meltFrameCount)+"px"):b.recycle()};this.recycle=function(){b.o.style.display="none";b.o.style.position=n?"fixed":"absolute";b.o.style.bottom="auto";b.setVelocities();b.vCheck();b.meltFrame=0;b.melting=false;b.setOpacity(b.o,1);b.o.style.padding="0px";b.o.style.margin=
"0px";b.o.style.fontSize=b.fontSize+"px";b.o.style.lineHeight=a.flakeHeight+2+"px";b.o.style.textAlign="center";b.o.style.verticalAlign="baseline";b.x=parseInt(g(h-a.flakeWidth-20),10);b.y=parseInt(g(j)*-1,10)-a.flakeHeight;b.refresh();b.o.style.display="block";b.active=1};this.recycle();this.refresh()};this.snow=function(){for(var c=0,d=0,e=0,f=null,f=a.flakes.length;f--;)a.flakes[f].active===1?(a.flakes[f].move(),c++):a.flakes[f].active===0?d++:e++,a.flakes[f].melting&&a.flakes[f].melt();if(c<a.flakesMaxActive&&
(f=a.flakes[parseInt(g(a.flakes.length),10)],f.active===0))f.melting=true};this.mouseMove=function(c){if(!a.followMouse)return true;c=parseInt(c.clientX,10);c<k?p=-2+c/k*2:(c-=k,p=c/k*2)};this.createSnow=function(c,d){var e;for(e=0;e<c;e++)if(a.flakes[a.flakes.length]=new a.SnowFlake(a,parseInt(g(6),10)),d||e>a.flakesMaxActive)a.flakes[a.flakes.length-1].active=-1;y.targetElement.appendChild(x)};this.timerInit=function(){a.timers=!A?[setInterval(a.snow,a.animationInterval)]:[setInterval(a.snow,a.animationInterval*
3),setInterval(a.snow,a.animationInterval)]};this.init=function(){var c;for(c=0;c<a.meltFrameCount;c++)a.meltFrames.push(1-c/a.meltFrameCount);a.randomizeWind();a.createSnow(a.flakesMax);a.events.add(e,"resize",a.resizeHandler);a.events.add(e,"scroll",a.scrollHandler);a.freezeOnBlur&&(i?(a.events.add(d,"focusout",a.freeze),a.events.add(d,"focusin",a.resume)):(a.events.add(e,"blur",a.freeze),a.events.add(e,"focus",a.resume)));a.resizeHandler();a.scrollHandler();a.followMouse&&a.events.add(i?d:e,"mousemove",
a.mouseMove);a.animationInterval=Math.max(20,a.animationInterval);a.timerInit()};this.start=function(c){if(w){if(c)return true}else w=true;if(typeof a.targetElement==="string"&&(c=a.targetElement,a.targetElement=d.getElementById(c),!a.targetElement))throw Error('Snowstorm: Unable to get targetElement "'+c+'"');if(!a.targetElement)a.targetElement=!i?d.documentElement?d.documentElement:d.body:d.body;if(a.targetElement!==d.documentElement&&a.targetElement!==d.body)a.resizeHandler=a.resizeHandlerAlt;
a.resizeHandler();a.usePositionFixed=a.usePositionFixed&&!t;n=a.usePositionFixed;if(h&&j&&!a.disabled)a.init(),a.active=true};a.events.add(e,"load",r,false);return this}(window,document);
//]]>
</script>

Como puedes notar, en el código hay letras rosa y letras naranja.
Las letras rosa (40) indican la velocidad de caída de los copos de nieve. De modo que si quieres una nevada suave, déjala en 40, pero si quieres una tormenta de nieve pon un número más bajo :)

Las letras naranja indican el color de la nieve. Personalmente prefiero el color blanco, pero no sé cual te gusta más, ponlo como mejor te parezca.

Disfruta tu nieve! No olvides abrigarte!

-Tortu