A little related to issue-216. Besides the text color and background color of the BadgeText, the extension icon itself is also a problem to adapt to various themes (Light/Dark/Custom).

Current state, developers can set static extension icon or dynamic change it by setIcon(). But:

• Developers can't set different icons for different themes (at least in static way). So the icon is good for one theme, but may be illegible for another theme.
• Single icon with transparent background is difficult to adapt to different color themes, unless you fill the background with a solid color.
• To setIcon() dynamically, developers may need to know current background color of the browser toolbar to choose a color with sufficient contrast to draw the icon. Note: background color of browser toolbar may differ from the system theme, see this discussion.