Package Manager's Filter Panel does not show borders when viewed on Windows

Steps to reproduce:

1. Open any Project
2. Open the Package Manager Window
3. Open the "Filters" dropdown panel and observe it's borders

Actual results: The Panel is borderless, it's hard to make out where it ends against the rest of the Package Manager's design

Expected results: Dropdown windows have borders

Reproducible with versions: 2023.1.0a1, 6000.0.75f1, 6000.3.16f1, 6000.4.8f1, 6000.5.0b9, 6000.6.0a5

Tested on (OS): Windows 11

Does not reproduce on (OS): macOS