I can definitely second the general sentiment.

Some time ago Joplin changed its default styles to add padding on the sides; this was undesirable for my purposes, so I went to revert the change and found out about the process, which involves hunting down classes in the dev console for each one of the several different editing components that Joplin uses.

The nitty-gritty details of the process are virtually undocumented, making even simple modifications opaque and time-consuming.