For your information, btrfs doesn’t have anything to do with whether the system boots in UEFI mode or in legacy BIOS mode. btrfs is just an advanced type of filesystem with a built-in volume manager. ext4, by comparison, is a more traditional type of filesystem.
As another FYI, there are many different theming aspects to KDE Plasma, i.e…:
- the Plasma style, which pertains to the look & feel of the panels and the desktop widgets;
- the panel arrangement and pre-population of the task manager with pinned icons;
- the window decorations and which window control buttons are located where in the title bar;
- the icon set;
- the cursor set;
- the fonts;
- the color scheme;
- the application style — whether it’s based upon any of the built-in themes (e.g. Breeze), or whether it’s set via
kvantum(which is an SVG-based theme engine); and - the wallpaper.
A global theme is essentially a bundle of all of the above components, complete with a script that arranges the panels and populates the task manager, and in such a way that the whole configuration can be applied in one go. However, nothing prevents you from installing any of the individual components while keeping the rest of your current theme intact, nor does the installation of a global theme force you to change your panel arrangement, wallpaper, and so on.
For that matter, you may also find this interesting… 
All of the above just as an FYI, of course. 