to change highlight text go to ACP>Templates & Style>YourTheme>than click on global.css or if no global.css found dont worry its yourtheme.css>in Selector: choose .highlight as i selected check snap.
check edited demo style 1
to change background color just change #FFFFCC to your theme color or any color you want.
check edited demo style 2
to add border just add this below to Extra CSS Attributes
border:2px solid #yourcolor;
you can also change highlighted text color by adding "color or change font or font size hope this help you.
if you using cloudflare you have to clear you global.css file cache to see the newchanges.