meequz created an issue (geany/geany#4249)

Currently I found no way to set a separate color for horizontal folding line. Only two parameters in color schemes exist related to folding the code blocks: `margin_folding` and `fold_symbol_highlight`. They do not influence the horizontal line.

Here on the screenshot is my custom color theme based on Darcula. I like the folded block to be highlighted with the horizontal line, but it's too bright, and it's impossible to make it less bright without making the main text color less bright too.