When I switch KDE themes only windows’ colors and decorations change but not bottom panel.
For example, I prefer light themes over dark ones. So I when switch to Breeze theme bottom panel remains dark. And no matter what theme I switch to panel remains dark.
How to change panel theme?
How to bind panel theme to global theme?
It’s called “Plasma theme”, and it is set in a different section of the Appearance page in System Settings. It is independent from the global theme, although changing the global theme will also change the Plasma theme.
If you do not find a Plasma theme to your liking in System Settings, then you can look for one at store.kde.org, but be sure to pick one that’s still being maintained. Plasma themes from 10 years ago are bound to break your system.
Also, an easy way to install it is to first install ocs-url from the AUR, and then you can install the theme directly from within your browser by clicking the OCS link on the theme’s page. 
To install ocs-url…
pamac build ocs-url
… or…
yay -S ocs-url
Shameless plug for my ‘follows your color scheme’ plasma theme:
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