The file toolbar_icon_source.svg contains the complete set of graphics in different layers. To create an individual icon out of it, follow this procedure:

1. Open toolbar_icon_source.svg, e.g. in Inkscape
2. Hide/show the layers needed to compose your desired icon
3. "Export" the icon to PNG, 36x36 pixel size (for Chrome/Edge)
4. "Save as copy" the icon as "Optimized SVG" (for Firefox)
5. Repeat for all desired icon versions

Note: The extension will choose the icon to show by filename, so make sure that is correct (take a look at the existing icon sets for reference).