This is most likely caused by an upgrade from a Mendix 8 or lower app that was using Atlas Core 2.
In Mendix 9 you have Atlas 3. If you’re coming from an Atlas 2 app, your theme folder is structured differently.
The Data Widgets app uses the “core” variables from Atlas 3 in it’s styling. The fix for me was changing 1 line in the file themesource > datawidgets > web > main.scss, Line 1:
1 @import "../../../theme/styles/web/sass/main";
Let me know if it helps.