Make keyboard more accessible

This makes the following things configurable via the config:

• keyboard background
• keyboard font size
• keycap text color
• keycap background (letters, return, other)
• text input background

Color selections were tweaked from the previous defaults to offer more contrast, taking inspiration from common color patterns on iOS and Android keyboards.

Hex string parsing was moved into the config class since it is now required in a lot more places.

Closes: #96 (closed)

This is how it currently looks with the changes from the MR:

Note that the colors are a bit different than in the screenshots in #96 (closed) since I confused [0-100] and [0-255] ranges when creating the former screenshots. Current values are color-picked from here with the return button color taken from here.

Happy to tweak the colors further.