Make keyboard more accessible (MR #105)

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