How can turn dashed line into fine line

Hello mpek,

please try with adding this CSS rule:

.cat-post-widget h2.widget-title {
border-bottom-style: solid;
}

If you use WordPress 4.7 or higher you can add the CSS rule with the customizer: https://kometschuh.de/Easily-adding-custom-CSS-using-the-WordPress-Theme-customizer.html

We’re nearly there! The color is not fitting well (too dark). I need a lighter grey like the other lines on site.

You should give this attributes and color a try:

.cat-post-widget h2.widget-title {
    border-bottom: 1px solid #cccccc
}

The best. Thank you!