Yes you can.
Styling Button objects
Button controls can be customised in a huge number of ways via CSS (Cascading Style Sheets). Rather than adding actions to set all of the possible properties, the Button object exposes a Set CSS style action. Knowing CSS will of course be an advantage here, but it is simple to use for simple properties. A few examples are listed below.
Make the text red: Set "color" to "red"
Make the background yellow: Set "background-color" to "yellow"
Make the border a thick light grey: Set "border" to "3px solid #AAAAAA"
Make the font bigger: Set "font-size" to "2em"
Make a dashed red border: Set "border" to "2px dashed red"
https://www.construct.net/en/make-games/manuals/construct-3/plugin-reference/button