(carefully) update to the new style

adapted from howto.rc3.world!69 (closed)

this addresses (part of) #2.

tbh, I found the new styleguide confusing, incomplete, and in some parts I outright disagreed with it. Things that are deliberately not as the styleguide would have them:

• I kept Montserrat as main font for normal text
• the navbar, sidebar, and links still have their old colours (the styleguide would set all of them to pure white, which — since normal text is also white — I find a horrible idea)
• the background isn't an image. Instead it's a shade of gray I got by blurring the 10c_Background_dark_theme textures until only one indistinct colour was left

Apart from that, things I did change:

• header fonts and style (now all-lowercase)
• logo (though it gets a bit squashed in the favicon, not sure what to do about that)
• section headings and text are all pure white now