The text style in the map bubble can be customized by modifying the HTML block containing the map code.
As an example, the following code in the HTML block makes the description in the bubble display in red. Note the “span” below:
<div id="iconDesc[@field:ID]" style="display:none"><span style="color:red">[@field:Description]</span></div>
Use standard CSS (Cascading Style Sheet) to format all or parts of your bubble text.