See progress here:
- Draft pull request (incomplete, just preliminary tests): ENH: Improve icon set by lassoan · Pull Request #5887 · Slicer/Slicer · GitHub
- Discussion: Update Slicer's icon set - #14 by lassoan
- Labs page: Modern Look · Slicer/Slicer Wiki · GitHub
We don’t plan to work on creating an icon theme (which would make it very easy to replace the entire icon set), so if you want you can investigate that and come up with a proposal (preferably as a drafy pull request that we can try).
You could also help with finalizing the icon creation process and creating icons using that.