You can do this by overriding styles that are created by the theme. For the Slide theme:
- Select the component in Xway
- Click on the + button in the Media Styles section of the Box Inspector
- Choose Descendants of Item from the Selector popup
- Replace the asterisk with .xb-slide-figcaption h1, so the Selector field reads #[Item ID] .xb-slide-figcaption h1 (don’t omit the initial dot in .xb-side-figcaption)
- Set a breakpoint
- Add a font-size property: font-size (name) and (e.g.) 12px !important (value)
- OK the dialog
- Add a second media style for the caption using the same steps, but this time replace the asterisk with .xb-slide-figcaption p
Note 1: #[Item ID] .xb-slide-figcaption h1 means "(1) h1 element that is a descendant of (2) an element that has class xb-slide-figcaption that is a descendant of (3) [Item ID] (where Item ID is the ID that the component has in Xway).
Note 2: If you change the component ID (using the Name/ID field in the Box Inspector), Xway will update the ID that is used in the selector.