Now testing script to change Plasma Theme opacity. Navigate to ~/.local/share/plasma/desktoptheme/spectrum-classic-basic/ and pass a value to the script between 0 and 99.
$ cd .local/share/plasma/desktoptheme/spectrum-classic-basic/
$ ./changeOpacity 40
Finally, switch to another plasma theme and back to see the new opacity.
This constructs the original Genome Dark theme with the new Spectrum theme.
Spectrum Dark Aurorae Theme
Spectrum 21 Aurorae Set
Genome Dark Color Scheme
Genome Dark Classic Plasma Theme
Spectrum Classic Plasma Theme
Spectrum Classic Opaque Plasma Theme
Spectrum Dark Icons
The Spectrum themes change color according to the Color Scheme you have equipped.
From System Settings, go to "Colors" to change your Color Scheme. Go to "Plasma Style" to change between Spectrum themes. Plain themes will change color with your Color Scheme.
To adjust transparency of the panel and menus:
Under Workspace: Workspace Behavior > Desktop Effects
"Background contrast" to toggle between 2 levels of transparency.
"Blur" will let you choose the strength of the blur effect as well as noise.
Under Appearance: Application Style > Window Decorations
Switch between the opaque and translucent versions of the window border.
"No Borders" and "No Side Borders" are the recommended window border sizes.
There are a lot of other effects in Desktop Effects to try if you are new to KDE Plasma.
Have you tried turning it off and on again? Sometimes plasma assets will not update and some may still be themed according to something you previously had set. This can usually be fixed by restarting the applications that used the asset (e.g. Dolphin).
To restart plasmashell, open KRunner with "Alt + F2" to execute the following: