Either you or the sender (probably the sender) has specified text as White. You often see this from folks that use dark themes and it never enters their head they are sending mail that is locked into their personal preferences.

But check in settings that you are not composing in white just to be sure.

I hardly believe an ordering system would set white text on a white background but I could be wrong ofcourse. What is the quickest way to reset both text and background color. They appear to be in two different places.

I suggest trying a different theme. It may be that the theme and the sender's HTML are in conflict. If you use an addon for looking at HTML in compose window, you can see what is actually there. For example, when using dark theme, I see white text in compose window, but If I review the underlying HTML, the font color is not being changed to white; the appearance of white is purely from the theme,