@Sampo2910 have you seen the article they recently released about supporting bootstrap 5 in v3 of NodeBB? This means that they will actively add functional support for bootswatch themes.
Whilst this is a welcome advance, several of these themes look quite ghoulish in my view, and lack creativity - essentially, they are simply recycling existing effort, and not bringing anything new to the table. I understand the approach in the sense that these are ready made and work very well, and why reinvent the wheel, but what would add much more value in my view is a fully skinnable interface where you could keep various iterations of color schemes that you liked.
It’s why I created the swatch engine in the first place.