I’m using transparent color (255,255,255,0) for my Content Box Background. The solution for me was to make the Hover Accent Color the same as the Icon Background Color, NOT the Content Box Background Color. I then used katieincognito’s CSS code to keep the Content Box Title color unchanged by changing the hex color code to black (#0000), which is the color of my Content Box Titles. I had to used Chrome Dev Tools to clear cache and reload before the CSS code changes took effect.
Please note that this CSS code replaces the on hover color for all Content Box Titles site-wide. If you need to target this code to a specific Content Boxes instance, use a CSS ID for the instance.