Change Views UI colors to be closer to the Seven style guide

Problem/Motivation

The Views UI defines a lot of colors that aren't used anywhere else in the admin interface. If Views UI is going to set colors, they might as well be close to the colors we use elsewhere. It would be better to move these colors into the Seven theme, so other admin themes don't have to override the colors.

Proposed resolution

Go through the colors defined in the Views UI and replace them with colors/styling with ones from the Seven style guide.
Move this CSS into the Seven theme

Remaining tasks

User interface changes

Visual tweaks only

API changes

None