It’s very easy.
Firstly, to JUST change the colour scheme is a bit messy - you need to update the configuration really.
For individual apps (let’s do add/remove programs, launch ‘add’ in the menu, or ‘pamac-manager %U’ in the terminal.
You can choose a theme for it each time by using 'GTK_THEME=Breath pamac-manager %U`
But for overall desktop, there’s more to think about.
There’s your Plasma theme (Qt)
There’s the GTK ‘Application Style’ - which gets used by Firefox and Chromium browsers.
There’s the terminal and the profile and theme and colour scheme for that… Lots of stuff.
Toolbars in Dolphin…
For all this stuff, there’s a Plasma widget called ‘PlasmaConfigSaver’.
So you can foollow a long Youtube video and set up a nice Nordic theme, then next week a nice Orchis theme - maybe one light, then one dark.
Before you start, save your DEFAULT using Plasmaconfigsaver.
Then do your thing and save your ‘Orchis’ theme, then do the light version and save ‘Orchis Light’.
Any time you tweak anything (e.g. change Terminal colours) you need to delete that profile and save it freshly.
To get it, right click desktop +Widgets then ‘get new widgets’ and search ‘plasma custom’ (because the name there doesn’t match the name of the widget… crazy).
I exported one here - not sure how well it will import but you can try: