Audit Colors on Theme Install Screen

[jorbin]

Colors are being used on the theme install screen that aren't in our less so they don't actually get changed to respect the users color theme.

I *think* the only four places that we use color that need to be put into less are:

• the theme count
• the feature filter
• the current filter underline
• the upload theme button

We should audit this page in general and make sure all colors can be transformed properly and it looks good with different schemes.

[helen]

I don't think the theme count needs extra attention on it, and the upload theme button seems to get colorized.

[ryelle]

I agree that the theme count doesn't need updating. Upload theme also doesn't need anything, the styling there matches the Add New buttons on Plugins, Posts, etc.

In 27522.diff​, I've updated the current filter underline (to the menu background color), and the feature filter (to the highlight background color). In the Light scheme, the filter underline is the blue highlight color, otherwise the line was too light.

[nacin]

In 27784:

Color scheme support for the new theme installer UI.

props ryelle.
fixes #27522.