Improve Views "Add Field" modal density and dark mode

[laryn]

The Views add field/filter UI needs some fixes and core overrides:

1. In dark mode, some core colors are messing things up:

2. The "Selected: " text (which shows up after selecting fields) is not populating with the field labels:

[laryn]

Here's a screenshot from Gin for Drupal:

I think part of the solution for the second issue may be turning off the toggle switch for these checkboxes.

[laryn]

A related core issue I filed: https://github.com/backdrop/backdrop-issues/issues/6355

[laryn]

I've made some tweaks in conjunction with that core issue above and I'll close this for now. If that PR isn't accepted I may need to come up with a plan B for the "Selected: " indicator at the bottom of the modal.