On the Button Text Settings, there is an option to choose the font type, size and color, however there is no way to add a text style (italics, bold, underline), but there is a workaround by adding a code to your button content.
Open the page for editing
Click the settings icon on the button or double click the button to slide out the button property settings
Click Button Text
Enter this code to the button content field to make the text BOLD:
<span style="font-weight:bold !important;">TEXT HERE</span>Enter this code to the button content field to make the text ITALIC:
<span style="font-style:italic !important;">TEXT HERE</span>Enter this code to the button content field to make the text UNDERLINED:
<span style="text-decoration:underline !important;">TEXT HERE</span>Replace
TEXT HEREwith your own button textTo combine styles, add the code and separate it with a
;(e.g.<span style="font-weight:bold; font-style:italic !important;">TEXT HERE</span>)