Gap between stair tread/risers and skirt
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!