Hey Mark,
The widget headings in the blog sidebar are controlled in the WordPress customizer. I took a look at the blog page you shared here, and I think you’re referring to the widget titles that are displayed in the sidebar (highlighted in red in the image below):

The Venetian theme looks like it’s using the alt-font CSS class on those headings, so they are able to be changed by going to the Fonts section then to Subheadings:

From there you will see the controls to change the font-family, font-size and capitalization of the widget titles in your sidebar:
