[BUG] Changing between light/dark themes does not load the correct Burp base icons.

[CoreyD97]

Describe the bug
Swapping between a dark and light them (or vice versa), does not use the correct dark/light icon packs. Instead, we must select the base theme first to load the correct icons, and then change the theme.

To Reproduce
Steps to reproduce the behavior:

1. Set Burp's theme to Light
2. Select a dark theme in Burp Customizer, like Dark Purple
3. Navigate to Repeater.
4. See that the icons in the top right are difficult to see, since they're made for a light background.

Expected behavior
Icon packs should be changed along with theme base.

Screenshots

Versions (please complete the following information):

• OS: Linux
• Burp Suite Pro / Community: Both
• Burp Suite Version: 2020.12