.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
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.