I would like the ability to change the default color for the text in the legend and pop-ups and other widgets from white to another color. I asked support and they said it is not supported at this time.
Out of the box, in the "builder" user experience of Web AppBuilder, you can change the color and font of the web app's title and subtitle only. See the Attribute tab > Branding section.
If you want to change the text color of the widget "labels" then you will need to use Web AppBuilder for ArcGIS Developer Edition and modify some underlying code files. But this will require some development skills.
Hope this helps,
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