also fix header that was wrong.
add styles for titles with backlink.
& other changes (see diff, i'm lazy)

also snippet that makes art menu is now separated as mixin (very
powerful thing, btw).

much of stuff included here is subject to change.

gray ones was kinda boring and also not recognizable as links. now
they're blue, but one can confuse them with titles and subtitles, so
maybe i'll change this later.

thumbnails are currently created using shell script and require
imagemagick installed on the server. i know it's stupid, but i think
that including thumbnails to repo is also stupid.

maybe later i replace this script with actual jekyll plugin, but for
this i'll need to familiarize myself with ruby.
but now i use following addition to the git hook on the server:

```
[ -f "$TMP_GIT_CLONE/art/thumbs.sh" ] && {
    dir="$PWD"
    cd "$TMP_GIT_CLONE/art/" && sh thumbs.sh && cd "$dir"
}
```

it happens after cloning to remote repo and before building with bundle.

overall, this is a bodge now, it recompiles all thumbnails every time
due to quirks in my current setup.

add two new categories - drawings and logos.

this css tinkering is so time consuming >_<

whooah!

there is some tough liquid-tagging involved.

i think that the pictures are just placeholders now. i will try to
improve this 'gallery' a bit later.

also, header is now fixed with transparent background.