Widgets are light gray in breeze-5.15.3-1.
Odd plasma widget look in breeze dark on rebuild plasma in build repo
Plasma 5.15.3 is not in the build repo. You mean this issue started after the Qt 5.12.2 update? Only affects one theme?
I'm sorry, yes after Qt 5.12.2 update. I try disable build repo and downgrade and check.
Don't. I'll rebuild all of Plasma on new Qt & rebuild frameworks. So far I tried to get away with just kwin, clearly not enough.
- Edited
I checked it anyway 
Yes, problem only with new Qt. And looks like just a problem with breeze dark, only.
Rebuild might not fix, already reported other places too, example https://bbs.archlinux.org/viewtopic.php?id=244988
But will do a full rebuild anyway, always better.
I've still not performed upgrade, so i'll wait for fix, since i'm using only dark themes.
Rebuild did not fix, so I'm afraid it is an issue for breeze dark & new Qt 5.12.2, not a distro/build issue.
If you disable composting, issue is gone.
Bug report is opened in KDE, please vote for it, so it gets the needed attention:
https://bugs.kde.org/show_bug.cgi?id=405548
One more update, fix is in review https://phabricator.kde.org/D19821, I won't post any further here now, once the patch lands, plasma-framework will be rebuild.
It looks correct patch, Qt's change https://code.qt.io/cgit/qt/qtsvg.git/commit/?h=5.12.2&id=dcc4e2c4d96a6ad1447b9fb679afca21bb381218
Patch is not correct/complete, only fixes kicker & notifications, not the tooltips, so not applying until the complete fix has landed in git.
It's landed in plasma-framework master.