You have done text styling for a field / section to display it either in bold / increased font. It is not working in Fiori / RUI client.
Fiori / RUI
Reproducing the Issue
In UI designer, for a text field, you have changed textstyle to Header1 as below.
Go to the work center Eg: Customers.
Click on Accounts view.
Then open any account.
Text field in HTML5 works as conifigured but in RUI / Fiori, such formatting is not working.
Text Styling is not supported in Fiori / RUI yet.
KBA , AP-RC-MOB-UIF , Responsive UI , Product Enhancement