Hi there!
Thanks for conatcting us.
You can use the following code in your custom CSS to change the underline color:
.sp-megamenu-parent>li:after {
background: red;
}
Please use your desired color code inside the above code.
And if you want to remove the underline color then use the following code snippet:
.sp-megamenu-parent>li:after {
background: none;
}
Note: Path towards custom CSS(Joomla 4): System>Site Templates Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
https://prnt.sc/5mBuSRh907N3
https://prnt.sc/ooiq-lkMEXr_
https://prnt.sc/nunlX-f5AhK1
Best Regards