Currently, it is not possible to customize the text we will set on an Expand Macro - you can set the text you wish to be displayed, but cannot set text color or background for the text.
It would be nice if this was a valid feature on the Confluence Cloud editor.

See the below image for reference:

And also:

• Change to global confluence cloud styling.
• Change default styling just for our site.
• Add options for us to customize ourselves.

Example: