Implement a fallback mechanism for icon fonts and SVGs used in CSS for high-security mode
It should be possible to fall back to PNG assets in high security mode where icon fonts (font awesome and tor icons) would not be available, and SVG graphics would also not be available.
I am not yet aware of a method of doing this reliably across all browsers, it may be worth talking to the Tor Browser people before looking at this to see if they have ideas.