Some useful code to style CSS block. You can add it to Cusom CSS box.
#1. Adding circle to end of Line
div.horizontalrule-block>div:after {
content: "";
width: 20px;
height: 20px;
display: block;
border: 1px solid #000;
border-radius: 50%;
position: absolute;
right: -20px;
top: 50%;
transform: translateY(-50%);
}

#2. Change Line Color in specific Line Block
Remember to change ID
#block-7b7f254e61fe276cdff3 hr {
background-color: #f1f !important;
}
#3. Change Line thicker
div.horizontalrule-block hr {
height: 5px !important;
}
#4. Round corners of Line
div.horizontalrule-block hr {
height: 25px !important;
border-radius: 10px;
}
#5. Change Line width
div.horizontalrule-block hr {
width: 100px;
}
#6. Add an outline color around Line
div.horizontalrule-block hr {
height: 10px;
border: 1px solid red;
}
#7. Add an outline with white padding around Line
div.horizontalrule-block hr {
height: 50px;
outline: 1px double red;
outline-offset: 10px;
}
