Most of the changes required for the dark theme

The only thing that remains is the Reddit icon. Waiting on @NSDex to
provide a version with a transparent background, and an inverted
version.
Increase specificity of Twitter selector

Also, looks like I forgot a few changes in the last commit.
Merge branch 'master' into dark-theme
Add inverted reddit icon
Merge branch 'master' into dark-theme
Merge pull request #298 from MediaCrush/dark-theme

Add dark theme