How do I make the site title fixed?

Try something like this:

#masthead {
position: fixed;
top: 0;
left: 400px;
right: 400px;
z-index: 10;
}

.content-area {
 margin-top: 160px;
}

It looks a little weird as the posts scroll up so I’m not sure a fixed header really suits this theme since the text is over a transparent background.

If you go this route, don’t edit the theme files directly, otherwise your changes will be overwritten when the theme is updated.

An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.

As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.