Part of #149

Right now there are some labels that don't achieve WCAG AA, which
specifies a text contrast of at least 4.5:1. The causes are varied, but
this commit fixes them all to exceed 4.5:1.

In particular, standardizing on an opacity level of 0.75 raises the text
contrast of those labels to:
- 5.73:1 on the opaque gray window background color
- 5.44:1 on a translucent background in Plasma with the current default
  wallpaper (though this will vary a bit with different backgrounds)

Now they meet WCAG AA.