Only hides system messages on the party screen, which is the only place they're seen 99% of the time. (The other 1% being in the Tavern, during world bosses. I'm not comfortable implementing the style in a way that it would apply to that without immediately being able to test that kind of thing.)
User JS installation is disabled for this style because I have not implemented it outside of the party screen--this means the conversion Greasy Fork automatically does can work if installed as a user script, but only if the initial load of Habitica is onto the party screen (otherwise, it fails entirely). I may allow JS conversion/download if I remove the party page limitation.
Special thanks to Alys for posing the question that got me thinking about how to do it!