Here is how I style my HR lines in CSS:
.line_height { height: 8px; }
.line_width { width: 100%; }
.line_hcenter { margin-left: auto; margin-right: auto; }
.line_vcenter { margin-top: 15px; margin-bottom: 15px; }
.line_color { color: blue; }
.line_bgcolor { background-color: blue; }
.line_bordercolor { border-top: solid blue; border-bottom: solid blue; }
Don't forget to apply these classes to the hr tag.
All three color-related classes (color, bgcolor, bordercolor) are necessary for a single colored line appearance.
The color and bgcolor classes are essential for ensuring consistent display across various browsers; otherwise, you might end up with duplicate lines on your page.