Problem: Steam popup boxes suddenly have zero text displayed in them.

Cause: Somehow the font got corrupted.

Solution: Changed desktop theme.
XPMCE --> right click desktop --> properties --> in Theme dropdown find a different theme. Windows then copies files & changes settings. Relaunched Steam and wallah -- text magically restored.

This article describes that my settings may have been "Admin Slowhacked" (probably a Team Fortress 2 server as that's the last Steam based game I played) and gives tips on how to easily correct/recover from that.
https://support.steampowered.com/kb_...1935-TIHK-2325

This article describes how to manually reinstall the corrupt or missing fonts in Windows. All my fonts were still in place so switching desktop theme seemed a better solution.
https://support.steampowered.com/kb_...1974-YFKL-4947

Hope it helps someone.