I just installed the new version of Bridge and it's gone backwards... again. A while ago I "me too(ed)" on a topic about the ability to change the background on a transparent PNG thumbnail to white whilst retaining the dark interface, which you used to be able to do, but got lost somehow. I got notified that the feature had been restored due to popular demand, so for the last few weeks, things were back to usable again.
Now, the independent sliders for transparent backgrounds are gone again - the closest to white I can get is about a 20% grey, AND... completely random, some .ai previews have a page background and some don't - in the same folder?!
Adobe 1 - My productivity 0
1> You can easily restore your 2018 or 2019 Bridge from A Time Machine Backup. Tell me you have a Time Machine Backup! If you don't know how I can explain in a reply.
2> The 2019 Bridge was so Broken it was 2 updates before it worked correctly and prior to that is was so slow I thought I was back on a mechanical Hard drive instead of an SSD! Not to mention numerous bugs in Photoshop & InDesign. With InDesign I don't update until March when an actual stable version is worthy of my production environment.
3> So I learned the hard way, I never UpDate any Adobe's Apps until 1 or 2 updates have fixed the dozens of bugs that make this whole subscription model a farce!
Try resetting your preferences
Using Transparency on/off feature, you can view the white background for the images.