General troubleshooting tip: Browsers offer some kind of "Inspect" function (at least, that's what it's called on Chrome) that lets you peek at how css is getting applied to any given element on the page.
Those arrows (when they're displaying in the sidebar seem to use the sidebar-foreground palette color. (At least, on
tiddlywiki.com, changing that bit on the palette is what makes the difference.)
So your cloned Nord-variant sidebar probably collapsed that value to that it's indistinguishable from the sidebar background...
-Springer