KDE plasma desktop themes
Information of the KDE plasma themes can be found from the KDE TechBase: https://techbase.kde.org/Development/Tutorials/Plasma
The KDE UserBase has a link to 'Using Multiple Plasma Themes': https://userbase.kde.org/Plasma/TweakingPlasma#Using_Multiple_Plasma_Themes
Plasma 5 theme editor before the plasma 5.6
The user can edit the plasma desktop theme from KDE system settings > Workspace Theme > Desktop Theme. In the Details tab there is an option make a cutomized/mixed plasma theme.
With the KF5 (plasma 5) the Customized plasma theme is saved to the ~/.local/share/plasma/desktoptheme/.customized .
The user can rename/copy (at this MyNewTheme) the theme to the ~/.local/share/plasma/desktoptheme/MyNewTheme .
After the user edits the metadata.desktop file...
the plasma theme is available from the KDE System Settings. The current theme has blue 'halo' ( depends current color settings)
Removal of the Theme Details KCM
KDE review board - Remove Theme Details KCM: https://git.reviewboard.kde.org/r/126953/
KDE Bug report: Bug 359127 - Desktop theme KCM 'details' and editing tab no longer present/functional - https://bugs.kde.org/show_bug.cgi?id=359127
Wishes - why there is no...
The KDE Forums ( https://forum.kde.org )has a sub forum for the 'User-submitted ideas for KDE desktop and software - KDE Brainstorm': https://forum.kde.org/viewforum.php?f=83 . There is also an option to file a wish to the KDE bug tracking system: https://bugs.kde.org