How to set the three buttons (enlarge, reduce, and close) in Budgie 26.04 to match the Apple style
These come the theme you are using - so certain vinceliuice themes have the CSS to make this happen. You’ll need to look at those github repos to understand the CSS changes and then apply them to whatever theme you are currently using.
In general, copy the current theme you are using from /usr/share/themes to ~/.themes and you can then make changes and apply these changes - again in general, switch from another theme and back again in budgie-desktop-settings to see the new CSS changes you have made.
For the labwc part - you need to create assets and config similar to vimix
For the CSS part - not sure - need to look through the src/gtk elements for the theme how the CSS is done.