Improve css

[Lab-8916100448256]

The custom css that is currently used is a simple copy'n'paste from the geekdoc theme documentation

This needs to be improved to have a better looking website.

Some remarks from @lockedmutex on matrix :

• make the header dark i guess, it looks good.
  

• I think font needs to be changed too
  Font looks like I opened MS Office 2007 on windows xp

[lockedmutex]

I can work on this, kinda got free time today.