chessmec: some colors are fixed, e.g. the red color of the amount of unread messages. will be not so easy i think

That would be rather trivial. Put the amount of unread message in a SPAN element, with a CLASS attribute - and say this attribute has value "unread-messages". In the default style sheet, you say:

.unread-messages {color: red}

This will make the amount of unread messages red. However, someone who wants the number to be yellow puts

.unread-messages {color: yellow}

in his/her stylesheet.