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.