MDL-52342 theme: Reduce specificity of selector

MDL-52342 theme: Reduce specificity of selector

These selectors were introduced to fix a real issue (highlighted by behat with
Chromedriver) whereby the 'Show section foo' link was partially covered by
another layer.

The fix in that issue was too broad and applied to all content > div areas,
which had some unforseen effects.

Since the original issue only affected activity instances, and sections (and
not content > div regions), I've removed this from the original patch.