This isn't a guide but it explains on what they do throughout PineDebug.
Thread below has informations.
Yes
This feature was probably requested twice but I was actually thinking about implementing them. Themes got added in v0.2.0 of PineDebug and they're community-creatable, they are for the flavor of style or if you do not like Carnell Blue all day.
PineDebug, since it's release, can have brand new buttons from other mods, starting with B. Carnell's Plus Pack! As long as PineDebug is a soft BepInDependency for the mod that has PineDebug support.
With it's following example code:

It does simply work, all it does is that it inserts a new button to the Stupid Casualties page and also adds in a new delegate for the /kill e button in order to properly destroy Mr. Portal Man's spawned output portals.
There is no need of patching the PineDebug manager itself (unless that said feature is unavailable) so add in PineDebug support if you need to.