How to set font size of the side bar title?

Hi there,

You can try setting it through the Widget Title on Appearance > Customize > Typography as shown here -
https://share.getcloudapp.com/Qwu4EAxR

Thanks.

Can you provide me css for underline for those side bar titles?

You can try this CSS if you want each character in a text string to have underline.

.widget-title {
    text-decoration: underline;
}

This means, even if the text wraps in 2 lines, all the texts will still have underline.

but if you want the element itself to have some sort of a line below the title, you can go w/ border-bottom.

Example:

.widget-title {
    border-bottom: 2px solid black;
}

Thanks Elvin.

No problem. :D