The global Spectrum theme for Strawberry Quartz. Allows configuration and features 2 neutral themes and one completely pink theme. The length of these names is getting out of hand.
Strawberry Quartz Color Scheme
Spectrum Classic Strawberry Quartz Plasma Theme
Spectrum Classic Colors Plasma Theme
Spectrum Classic Plasma Theme
Spectrum Folders 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, though you may need to reapply them.
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.
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 to something you previously had set. This can usually be fixed by restarting.
To restart just plasmashell, open KRunner with "Alt + F2" to execute the following 2 lines: