Images now have a default maximum width of 100%.
Generated tree images now must be marked with class `gentree' and will
be color-inverted (i.e. their black parts will be white, while the
transparent background remains transparent) with dark themes.
The TOC and the language dropdown have been inverted.
Replaced all text in navbar by icons, moved TOC to the right of Home
button. TOC is now correctly displayed with small animation. Added black
theme to light and dark themes.
