Per the style guide:
ProGuide Style Guide · Esri/arcgis-pro-sdk Wiki · GitHub
I am using
<TextBlock Style="{DynamicResource Esri_TextBlockRegular}">
to make the text white in the dark theme and black in the light theme. But the text is dark gray in both themes.
Furthermore, the doc says
Certain UI elements such as TextBlocks, radio buttons, etc do not need any styling. ArcGIS Pro will automatically style them using the "Default" style.
but if I leave out the style altogether the text is just black in both themes.
I cannot reproduce Ted. Somewhere the style or brush is getting overwritten. If you'd like me to debug you'll have to get the Add-in code to me
Just closing this up. After some DM'ing, Charles Macleod and I determined that due to the age of this project (1.3, 2016) it would be best to recreate the project from a new project template in Visual Studio. There must be some legacy stuff preventing the theme-switching styles from working.