Only other things I can think of to try would be removing the indicator plugin from the panel & then adding it again.
Maybe try creating a new panel with the dark theme already in use instead of switching from light to dark with the same panel.
Creating a background image & using that on the panel.
Combinations of the above.
Interesting that using a solid colour didn't sort it, I can't remember if I had to create a new panel to get it to work, or whether I just kept playing with themes until I got one that made the indicator background dark like I wanted.
But I do remember having to play around with things to get it to go dark as I wasn't too impressed with it staying white & the rest of the panel being dark.
Ambiance-Dark style is the one in use on my panel pics above.
Maybe try creating a new panel with the dark theme already in use instead of switching from light to dark with the same panel.
Creating a background image & using that on the panel.
Combinations of the above.
Interesting that using a solid colour didn't sort it, I can't remember if I had to create a new panel to get it to work, or whether I just kept playing with themes until I got one that made the indicator background dark like I wanted.
But I do remember having to play around with things to get it to go dark as I wasn't too impressed with it staying white & the rest of the panel being dark.
Ambiance-Dark style is the one in use on my panel pics above.