I misstated the report. The report says most browsers, not most sites, have an option for turning off colors and background images for people with certain kinds of vision problem. The problem is that XF edit control icons use background images, so when colors are turned off, the edit control icons go blank. Here is a screenshot of this. I installed an addon to Firefox to check this out
For kicks, I took a look at tinymce and with colors turned off, here is what you get:
This is really a problem with the Firefox add-on. It's removing any "background" image which is an incredibly common technique and is used here. There isn't really anything we can do about it (we can't really support browser extensions that make arbitrary changes to the page HTML/CSS).