I don't think this is possible within QGIS. So I suggest you export your map to Inkscape and edit it manually.
Once you're done with all other things you want in the print composer , export your map as SVG then open the SVG with Inkscape.
Inkscape is a free and open-source graphic design software. So it blends seamlessly with QGIS.
You can download and learn ...
There's no "render as html" support for legends in any way (yet).
If you want to build a legend with html you need to use a html frame.
If you want graphics you could combine a legend without any text with a html frame styled the way you want.