Rollup merge of #58303 - GuillaumeGomez:stability-tags-display, r=QuietMisdreavus

Improve stability tags display

The issue was the font color on dark theme. Fixed now:

<img width="352" alt="screenshot 2019-02-08 at 14 15 24" src="https://user-images.githubusercontent.com/3050060/52483276-bd810380-2bb3-11e9-8d46-95368569ac85.png">

r? @QuietMisdreavus

diff --cc src/librustdoc/html/render.rs
Simple merge

diff --cc src/librustdoc/html/static/themes/dark.css
Simple merge

diff --cc src/librustdoc/html/static/themes/light.css
Simple merge