One of the things which makes Plasma so attractive is the officially supported option to customize also the style, and that beyond colors and wallpaper, to allow users to personalize the look to their likes. And designers have picked up on that and did a good set of custom designs (store.kde.org lists at the time of writing 454 themes).
Intended changes and unintended changes
In the 11 years since Plasma was released first (January 11, 2008) there has been some evolution of the theming options. Sadly also sometimes regressions in the support of older options were introduced, mainly in the early phase of Plasma 5 when code was ported to QtQuick 2 and the new default theme Breeze did not expose the regression or even triggered some changes for its special needs or for simplifications it allowed.
Given Breeze as default was pleasing to most, and possibly some alternative theme designers were lost during the longer phase from a pleasing KDE4 to an again pleasing Plasma 5, and new theme designers made their themes directly matching latest Plasma 5 Theme support, the piled-up regressions were not a real issue for those contributing.
Well, not a real issue unless on started to restore an old theme like “Fluffy Bunny”. And was wondering why it looks broken. And then wondering the same while playing with all the other old themes still available on store.kde.org.
So much partially good artwork being broken made my heart bleed, and in the absence of own designer abilities it was time to document the regressions as well as fix some code where possible to restore previous variability.
Teaching Porting to Plasma 5
So, theme designers, please find a new page on techbase.kde.org about Porting Themes to latest Plasma 5. It lists the changes known so far and what to do for them. Please extend the list if something is yet missing. One might test the fixes in the code against one’s themes with latest development versions of Plasma using Live images.
Fresh old Air & Oxygen
And even the Oxygen & Air themes, the default Plasma themes during KDE4 times, while being maintained as official themes part of current Plasma releases, had suffered some small regressions (e.g. analog clocks hands rotation point or progressbar height). No more, with Plasma 5.16 the visual experience of Oxygen & Air should be closer to the original.
Luxury on the borders results in bad looks
Sadly there is one thing which is not so easy to fix: Plasma code now assumes for any frames (like with borders of panels & pop-up dialogs) that the actual border visuals are negligible and the margin from the outside to the content is the same, with border on the side enabled or not. With luxurious decorated borders (like Fluffy Bunny or Spoons Original) this sadly results in bad looking bigger margins on sides where the border is disabled (see details). Which is sad especially given that High DPI display actually would allow more fanciness, even more when in the future Flat design gets replaced by whatever new trend. My passive dreams & hopes are on for Plasma 6 😉
Separating Plasma 5 compatible themes on store.kde.org
And while there now is a list of things to do to make old themes work again, many themes on store.kde.org no longer are maintained. There is talk about separating those, e.g. at time of Plasma 5.16 release, so there would be a clean list of working and maintained themes for latest Plasma 5.