Laanes wrote: Any advice how
Any advice how to improve it or make it more usable by using CSS is very appreciated
Wrong question, really. First you have to fix your html, which is, I'm afraid, quite horrible. You have over a hundred validation errors in your html. The CSS rules do not apply to invalid html.
And you are using horribly out of date coding practices as well, particularly using tables for layout. There are other very bad practices there too, such as not putting your menus in lists. You need to learn what semantic html is, and why you need to learn to use it.
The CSS is beside the point here, really. You are worried about the color of the walls of your house while ignoring the fact that the foundation is rotten.