On recent betas that feature the new Darkest / Dark / Light Appearance modes, the API available to third-party plug-ins to determine the background color of the inspector is no longer working. It appears to be retuning black, on all three settings.
The PFAppSuite4 PF_AppGetBgColor function is returning the RGB values 0,0,0 instead of the actual background color of the window.
This regression breaks any plugins that rely on that API to know what color to fill custom UI in. For example, here is the custom UI of a plugin in the release version of Premiere Pro:
Release version of Premiere, where the API works
This is how the same UI looks in the latest beta, where the API is returning solid black:
Beta version, where API returns 0,0,0 on Darkest
...the problem is unaffected by the current Appearance setting. Here is the same UI drawn in Light mode on the latest beta:
Beta version, where 0,0,0 is returned in Light mode