How do I customize the icons in Widgets?

Summary

There are some Icons in the Widgets that are too small and hard to see.

Content

The Course play Icon is very small and can be hard to find.  This is causing issues with my users as they can't find how to start the course.  Also on the My Team Monitor the Management Report icon has the same issue.  you don;t realize it is there unless you already know about it.

How can I make modifications to these icons?

Thank You.

Samantha