Improve contrast for dark themes

[krassowski]

Fixes #41

File editor before:

File editor after:

Inactive cell before:

Inactive cell after:

Disclaimer: I do not see all the colors and it is just ok for me now (but the contrast could be even stronger). If I overdid this please reduce accordingly to your liking.

[krassowski]

Maybe alpha 0.3 would be enough. I do not know. This is tricky because increasing it too much reduces contrast with text. I think the reasonable range would be 0.25-0.4.

[krassowski]

I added dark text-shadow to improve contrast:

[ijmbarr]

Thanks!