Using the Chrome ColorZilla plugin we can identify the color as #229AD6
.
Using the browser's Developer Tools we can find where that's used.
Figure 1. The theme's definition after I had overridden it.
Add the CSS below into your Project Browser | Perspective | Styles | stylesheet.css. (Right-click, Advanced Stylesheet, if you haven't turned it on yet.)
:root {
--callToAction--hover: #229AD600;
}
It overrides the theme's color with the same color but with transparency set to 00 (in the 7th and 8th hex character).
This will work with light / dark themes automatically.
Note that the color variable may be used on other components.