For those who want to customize further with their own CSS in userChrome.css with !important rules, here is a growing documentation of main ids and tricks, sorted by category (ids and howtos for now).

See also #112, https://github.com/Aris-t2/CustomCSSforFx/issues/109#issuecomment-402849175, and https://github.com/Aris-t2/CustomCSSforFx for more.

Contribution: if you want to contribute or correct things, this wiki is now editable.

Ids

.nofavicon

HTML class of the nofavicon favicon
E.g. for changing its image:

.nofavicon {
  background-image: url("path-you-want/folder-img-you-want.png") !important;
}

#mglass

HTML id of the magnifying glass button
E.g. for changing its image:

#mglass {
  background-image: url("path-you-want/folder-img-you-want.png") !important;
}

.twistiena

HTML class of the twistie for an empty folder
E.g. for changing its image:

.twistiena {
  background-image: url("path-you-want/folder-img-you-want.png") !important;
}

.twistieac

HTML class of the twistie for a non empty folder which is closed
E.g. for changing its image:

.twistieac {
  background-image: url("path-you-want/folder-img-you-want-when-no-hover.png") !important;
}

.brow:hover .twistieac, .selbrow:hover .twistieac {
  background-image: url("path-you-want/folder-img-you-want-when-hover.png") !important;
}

.twistieao

HTML class of the twistie for a non empty folder which is open
E.g. for changing its image:

.twistieao {
  background-image: url("path-you-want/folder-img-you-want-when-no-hover.png") !important;
}

.brow:hover .twistieao, .selbrow:hover .twistieao {
  background-image: url("path-you-want/folder-img-you-want-when-hover.png") !important;
}

.rtwistieac

HTML class of the indicator at front of a folder in search window
E.g. for changing its image:

.rtwistieac {
  background-image: url("path-you-want/folder-img-you-want.png") !important;
}

Howtos

Override minimum and maximum size of Firefox sidebar; to give more (or less) room to BSP2

Thanks to @Gitoffthelawn in https://github.com/aaFn/Bookmark-search-plus-2/issues/170#issuecomment-1022911448

#sidebar-box {
  min-width: 0 !important;
}
    
#sidebar {
  max-width: 100% !important;
}