Unfortunately, there is not yet a configuration option to allow users to change that color. As mentioned in the Hide Fold Lines discussion, there may be messages that plugins can send – that topic had an example of hidden lines, but I was able to find an equivalent for the fold line: the PythonScript plugin could send
editor.setElementColour(ELEMENT.FOLD_LINE, (0,0,255,255))where the color is (RED,GREEN,BLUE,ALPHA) where ALPHA should be 255
here’s the results of me running that command in PythonScript 3:
dd61ca72-575e-4a11-92d2-440a75a20b09-image.png
and then I changed it to GREEN instead of BLUE:
editor.setElementColour(ELEMENT.FOLD_LINE, (0,255,0,255))cd974739-3173-49bb-b3f3-8859f56ffe3b-image.png
And hiding it completely can be done by either setting the ALPHA to 0, or editor.setFoldFlags(0) in PythonScript (or the equivalent, as shown in that other discussion, in NppExec or other such plugin)
This feature was requested in 2023, but neither the developer nor any other volunteers have apparently put in a PR to implement it. I’ve just “bumped” the discussion, with links to a few related discussions here; if you’d like to go upvote that, and/or put in your 2cents, that might encourage someone to put in the effort.