Thank you for your answer. Just want to clarify something
On my pc, I see that there is an option to change specifically the outlook theme.
It's in Tools->Options->Mail format->Stationary and fonts->Theme.
I looked in Tools->Options of the outlook on my device (windows mobile 6.5.3), and there's no such option there (there is very little amount of features there at all).
Is there some upgraded version of outlook for 6.5.3 that has this option, or this is it?