I have a file with a custom theme. I found and removed all the instances of local css using the XML version of the DDR. When I add a new field via the Field Picker, the text label for the field comes in as a modification to the Default style. The field itself appears fine.
When I drag a new field onto the layout via the Field Tool in the menu bar, the text label correctly associates itself with the custom theme Default.
Here is what it looks like with the Field Tool:
And here is what it looks like with the Field Picker:
• Unhosted file
• Mac OS Sierra 10.12.6
• Mac mini (Late 2014)
• FileMaker 16.03.3.302