Added "eReader" theme with improved contrast for e-ink screens

Fixes #87

I found Mikupad shows up very badly on e-ink tablet screens due to all of the themes having non-white backgrounds.

So I added a new theme called "eReader" which:

Sets the background to pure white and the text to pure black.
Used font-family: monospace as this seems to slow up with the crispest edges.
Used font-size: 16px as this too seems to be slightly crisper than the default 15px (likely getting interpolated).
Changed to use --color-miku: #08f for slightly better contrast.
Made the prompt-overlay use slightly less transparency for much better contrast.

Overall it looks much clearer now and even on greyscale screens the colourised-overlay is mostly interpretable.

lmg-anon / mikupad