In general you can change the CSS by creating a file
userstyle.css[/m] in the folder [m]conf. It will not be present, you have to create the file on your own.
Change the border style of the hr element to change the look of the horizontal line.
This code will give you a red horizontal line:
hr {
border-color: red;
}
This code will give you a red and dashed thick horizontal line:
hr {
border-top: none;
border-bottom: 5pt dashed red;
}
Also see
https://www.w3schools.com/tags/tag_hr.asp and
https://www.w3schools.com/cssref/pr_border.asp.
Kind Regards,
Lars