Some questions from a newbie of customization

[birlest]

Can I find somewhere some different sets of colours in order to change the original ones?

Once I decided the new colours, how can I set some simple different layout of colours (no more), from which I can choose (a sort of menu) in the Admin CP. without manually changing them with the CSS selector... and possibly let the user choose too (like the layout at the bottom of this forum)?

Is it always possible to simply revert the decision?

thanks