Yes... this is beyond what the "complex" fltk widgets will support. The "fast and light" ethos means these widgets don't have any way to do that really... I'm not sure about Fl_Help_View, but for Fl_Text_Editor, you'd basically have to derive your own class, then implement the ::draw and ::draw_*** methods yourself (copying the draw_*** code from Fl_Text_Display) and create a suitable background surface that way.
But even that would be a lot of work - there are several ::draw_*** methods involved, several of which will overwrite the background (text strings in the fltk text editor have both foreground and background attributes) so it would be hard to make that work out well, I suspect.
I'd speculate that, if you need that sort of visual effect, one of the more elaborate GUI toolkits might be your better choice, TBH, as they may have that ability already.