The little turquoise line is coming from here.
#secondary .widget-title::after {
background-color: #009688; /* remove this line */
content: "";
position: absolute;
width: 50px;
display: block;
height: 4px;
bottom: -15px;
}
In order to remove the turquoise line you need to either override or remove this line:
background-color: #009688;
.
You can do one more thing. Increase width of the turquoise line so that it takes the width of widget area.
#secondary .widget-title::after {
background-color: #009688;
content: "";
position: absolute;
width: 85%; /* Width increased to 85% */
display: block;
height: 4px;
bottom: -15px;
}
However, either way you go, as mentioned by @addweb-solution-pvt-ltd, you better use a child theme otherwise updating the theme will erase your changes.
Hope this will help.