Tweak Umami styles for the media library view

Problem/Motivation

While the media library is functional when using Umami, there are a few styles that need tweaked:

1. Button styling is slightly off for buttons rendered in jQuery UI widget. Specifically the button text is black instead of white.
2. The "Add media" button is a bit too large in the modal. It would be nice if it was smaller.
3. The "Apply filters" button should be center aligned since Umami exposed form fields aren't the same height.

Proposed resolution

Fix any problems we find. If you notice items jumping around on hover, that's resolved in 🐛 Style the media library widget when used with off the canvas dialog Closed: outdated .

Before patch:

After patch:

Remaining tasks

Review the patch.

User interface changes

Some styles will be tweaked when using the media library.

API changes

None.

Data model changes

None.