Fix #714 - Menu styles + font sizing issues

[bryphe]

Address a couple of issues with the menu:

• Use the foregroundColor and backgroundColor from the current color theme, and no hardcoded colors.
• Use em instead of px for font-sizes here, since px doesn't work correctly for HDPI displays. We should use pt as the font sizes.

Remaining issues:

• Update default font size to pt on Linux/Mac

[bryphe]

Fixes #714

[bryphe]

New look against the default onedark colorscheme:

And in solarized8_light:

Not perfect, but it is better than before, and an incremental step. Once there is more thorough sync'ing of the colorschemes, we'll be able to do a bit better with the highlight colors.

[bryphe]

(Moving remaining fix - updating default font sizes - to a separate fix to improve font rendering).