This will work much better for you current snippet!
.CodeMirror-activeline-background {
background-color: #e3e7ef;
}
.CodeMirror-activeline .cm-hmd-list-indent .cm-tab::before,
.CodeMirror-activeline .cm-hmd-list-indent ul ul::before {
width: 3px;
bottom: -2px;
background-color: #0000ff;
}