Allow Custom Theme-ing

[zwimer]

Description

As mentioned here https://github.com/angr/angr-management/issues/808#issuecomment-1333155739 while we presently permit users to override theme colors, those changes may not persist.

It might be nice to allow changes to persist by saving a new theme (something like 'Custom 1') when a user changes a color on one of the preset themes, saving this into the angrmanagement config.