.0 versions can contain bugs not found in the pre-launch versions. You can help us get them removed by reporting them. And look out for updates to Plasma, as they will come with many fixes over the next few weeks!
As for the themes and widgets created by third parties, we have already publicly advised developers to port their stuff. We have two pages available to guide them, one for themes, one for widgets.
If you, the user, discover your favourite theme or widget is failing, please look for it in the KDE Store and notify the author, as KDE has no control or leverage over third party developers.
Thank you
That requires installing the debug symbols, right?
Not necessarily. If you go to https://bugs.kde.org, you can just describe the bug in your own words: what you do to trigger the bug, the expected result, what really happens, etc.
Of course it is best if the bug is reproducible, i.e. that you discovered that every time you do x, y and z, a crash happens, for example. Like that, devs can confirm that it is indeed a bug of the software and not something that happens only due to some special configuration or hardware problem.
Some distros (Fedora at least) have debuginfod integration,meaning that gdb will automatically download debuginfo files. The downside is that it will take a while for download to finish (for the first time at least) since it gets symbols for all linked and loaded libraries recursively, and it will take a few gigabytes in ~/.cache.
It may help getting backtraces in DrKonqui which are needed if there are underlying bugs that are not visual, like very slow startup, crashes, out of memory bugs (memory leaks) etc.
Also running
journalctl -f --priority erron startup can help. Normally there should be no error messages, although this never happens.