To the left of the theme widgets in a vertical stack are the Heading Colors widgets.

You can assign background colors to each of the <h1> to <h6> tags. To do this, select one of the dash_hn notes, open it, click the color well, and pick a color. (Notes with empty colors will appear black and their corresponding heading will remain transparent.) To activate this feature, open the Colors: OFF/ON note and toggle the checkbox.

Headings will export with the assigned color as a background-color fade from left to right.