Hi,
Using Qlik Sense February 2021 Patch 2 and the Date picker from the Qlik Dashboard bundle.
In the Hub: When the date range dialog is displayed the characters are displayed in a nice san-serif font.
When the sheet is accessed using an URL (Single Configurator) or in iframe: The character font seems to ignore the theme and displays the characters in "Times New Roman" font.
Actually I use my own theme, but this behavior is the same for the standard themes. It is ugly.