Try the below in userstyle.css.
mark {
font-size: 12px;
padding: 4px 10px;
background-color: rgb(244, 245, 246);
color: rgb(115, 133, 152);
border-radius: 100px;
border-width: 0px;
}
So that they match, the settings have been taken from the css for the tag buttons (light theme).
Using the above the code ==highlight== becomes 
If it is only the shape you want to change use:
mark {
padding: 4px 10px;
border-radius: 100px;
border-width: 0px;
}

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