How to change style of underline on H1 font

To change the style of underline on H1 font, you can do these.

#1. First, find the ID of the block. Use below tool.

In my example, we have:

• #block-yui_3_17_2_1_1722898813189_4140

#2. Use this code to Custom CSS box

#block-yui_3_17_2_1_1722898813189_4140 h1 a {
text-decoration-thickness: 5px !important;
text-decoration-color: #f1f !important;
}

#3. Result