Fading Led (leds) inside computer looking like heart beat. Answered

Hi!

First of all I want to apologize my English but its not my mothers language.

So what I would like to do is add warm white led (leds) into my computer what would fade out and fade in like really slow pulse. How this could be done.

I can do basic soldering and schematics but the easier the schematic is the better :)
And as cheap as possible because I'm not that rich at the moment.
I have googled this a bit but found so many different versions and also they all faded so fast and I want my led (leds) to fade slowly. And because the idea is to but them to my computer this project would be using either 5 or 12 volt source.

Thanks for all who can help me!

Make