Root [Verizon] What do i need to know about themes?

[rsarno]

I have never touched a theme, but i see many people discuss it

Do i download them from the market? Or are they all sideloaded? Where do i find them?

Also do they only work on stock JB rom's? or will they work properly on most of the ROM's i see? What do i need to know in order to determine if it will work, or break stuff

thanks!