My stairs have a gap between the skirt board and the treads and risers on one side. They are tight fitting at the bullnose of the steps but then as you get farther back to the riser there is a wedge-shaped gap. From below it appears that the skirt board has pulled away from the treads over time. The stairs are supported by two thick stringers, so I don't think this is a big structural issue as much as an aesthetic one. My plan of attack was to shim between the treads and stringers to ensure that they were well-supported. Then I can use a gap-filling compound to fill in the gap between the treads/risers and the skirt board. Originally I thought about trying to pull the skirt board back to be tight with the treads and risers but I am thinking that might be hard/impossible without major work. The treads are housed so I can't pull them up easily to fill the gaps.

Does anyone have any advice or suggestions on how to do this the best? I'll have to use stainable/paintable filler because I was planning on staining the treads and painting the risers white.

Thanks for your help!

If you use stainable filler, tape the edges of any adjacent wood you want to stain, because any smudges of filler that get onto the wood will show, even after sanding, and it takes the stain differently than the surrounding wood. Also gel stain will blend the filler into the surrounding wood better than most other stains. It's a pain.

You say the treads are housed--to me, that means that they fit into the stringer and trim board--your balusters do not rest on the tread, but on the side trim of the stairs in a straight diagonal line, like at the top of the railing.
I was picturing the gap as being visible from the top of the tread--that it didn't fit into the trim all the way from front to back--and I couldn't see how the front could fit on every step, but not the back of each?

If the treads are open (visible) on one side, with the balusters resting in pairs on each step, then the gap is zero at the front, but wider as you approach the front of the next step, so the gap runs horizontally? It would seem to me that if every step is that way, then they were either poorly done to begin with, or the outside trim was replaced at one time, and the cuts were off.