I've reproduced the problem. The config conversion process is changing some things in the Light colors that should only be done to the Dark ones, if the Opus 12 colors looked dark.
We've fixed this for the next beta, in terms of what happens when loading an Opus 12 config.
If you want to fix a config that has already been converted, a very quick way is to double-click the Default Light theme under Preferences / Colors and Fonts / Themes.
Thanks for finding the problem and letting us know!