There is a CSS style selector (.i-section-heading-text) you can use to modify the default style of the Collapsible Heading Widget heading text.
To create a style rule to customize the Collapsible Heading Widget heading text:
- On the Project Explorer open your Stylesheet (located under Other files, and named customstyles.css). If your project doesn't already have a stylesheet, click New Stylesheet on the Project Ribbon Tab;
- With the Stylesheet editor open, right-click the Class node on the Stylesheet Explorer;
- Click New Rule from the displayed context menu;
- Select Named Style and type SectionHeadingText and the name;
This will create a new style rule (with a selector of ".i-Section-Heading-Text" in which you can set Style Attributes as you wish (e.g. customize the font size, color etc.).