This one has driven me nuts for years. You create a menu for your main screen with several videos on it. When you mouse over each video, it highlights in this horrible drab yellowy color. Looks like Hep C. How do you change that mouseover highlight color?

Change the color in the Fill color in color set one.

This is why I always change the Highlight mode to Underline.
If you want to keep the rectangle though, click the Color Sets tab in the Properties window on the right side of the screen.
Now change the colors on Color Set 1 to whatever you want.

I often use the text rectangle, but I change not only the color but also the transparency level. I choose the color used based on the background of my menu.

I most often use light white (white with transparency set about half) or a grey.

Like Mike I used to just change it to underline, but I currently prefer the text rectangle.

This is why I always change the Highlight mode to Underline.
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Where is the button that switches to underline?

It is under button properties>highlight>style