Add styling for media library

Problem/Motivation

The module already adds a lot of improvements to the media library for frontend themes to make it look the same as the backend. I did find a couple of issues though:

1. The media type menu (when multiple types are allowed) was not styled.
2. The media library add form looked different from the backed.
3. The dialog button pane did not align properly and was too big.
4. The second step in the add form was not yet styled.

There might be more little issues when you look closer, just fixed the really obvious issues. The changes are already big enough as is.

Before:

Steps to reproduce

Enable the gin_lb module in a very basic theme (for example stark) and check the result in layout builder.

Proposed resolution

Add the missing styles and templates to improve styling.

After:

Remaining tasks

User interface changes

API changes

Data model changes