@dangeruss It is possible. You will have to either fork persona to add in your changes or make a new theme.
If you need an example of how custom skins can be applied to the theme you can check these 2 themes.
https://github.com/pichalite/nodebb-theme-slick
https://github.com/pichalite/nodebb-theme-material
You can even follow these examples to add a modified version of an existing bootswatch skin.