508 remediation of Storyline course

Hello, 

I am working on a 508 remediation project, and this is my first time doing this in Storyline. The original project was created as a single scene, 652 total slides. When I ran the accessibility checker the first time, results showed the project had over 10,000 errors. I have slowly been chipping away at this, but I do not know if I am doing the work in the most efficient way possible. I've split the course into scenes based on the content, and I'm working my way through each slide individually. Is there a way to make universal changes to parts of a course? For example, all of the X icons to close a window in a layer need alt tags and enlarged to be 24x24. Is there a way I can put this in so that anytime that icon appears, the necessary changes are made? Or do I have to go through each slide?

The course has other issues (no unique slide titles, fonts too small, color contrast not compliant, even the initial font used is not accessible), but if I could solve even this one issue, it would save me some time. 

Thanks for your help!