Uh... What makes this better than just installing something like the Stylus extension and learning some CSS? For the price (and even a subscription?) it just seems ridiculous and just spending some time with perusing MDN docs and/or an LLM. We may not even know that in the future that this extension will be properly maintained for a long time, making the settings difficult to migrate. Will this export to CSS?
Even if this is aimed towards non-technical users, in the first place most laypeople looking to change websites' looks will either (a) use Arc Boosts most likely, (b) learn some CSS eventually since they will most likely be design-oriented people, or (c) use Stylus if they have heard of it and just use the preexisting styles made by the community on popular websites that a majority of people spend time on (which already have good styles)...
[end devil's advocate]
The UI of the extension looks nice though but it just seems kind of ridiculous with the pricing.