Achieve WCAG AA text contrast for labels
Part of plasma-desktop#149
Right now we use opacity to reduce the visual intensity of subtitle labels. We tend to use nonstandard opacity levels, and the levels we do use never achieve WCAG AA, which specifies a text contrast of at least 4.5:1. Some of these labels only get to 2 or 3.
Standardizing on an opacity level of 0.75 raises the text contrast of all these labels to:
- 5.73:1 on the opaque gray background
- 5.44:1 on a translucent background with the current default wallpaper (though this will vary a bit with different backgrounds)
In addition, use the semantically correct text color for text fields, which should be opaque but use the disabled text color.
In all cases, text components provided by libplasma now meet WCAG AA.
The changes are quite subtle and I imagine most people will barely notice, so no before-and-after screenshots.