Add the ability to style column groups (then rows) either in the Customize tab (maybe under Row color or a tab) or under Column settings > Conditional formatting. In some instances your plugin users are in multisite environments where custom CSS is blocked for security reasons but the other settings (under Customize and Conditional formatting) are allowed. The default styling of the column group blends in, visually, with other cells and making it difficult to pick out the grouping (the text isn’t bold and the background is the same as other rows).