Text contrast (particularly for `Show next code `)

[kojq]

Show next code seems to generally have poor color contrast for the codes, regardless of theme. I use System default (AMOLED). Perhaps more amiable palettes can be chosen.

Furthermore, I believe that the contrast becomes even worse when Tap to reveal is enabled, presumably taking on the color of the text placeholder color.

I'd appreciate both aspects being adjusted.

#696 seems tangentially related.

Thanks!