I’m using omniplan 4.5.1 for Ventura 13.0.1 on M2 mbp. I cannot change the font of the item title.
The best way to report bugs is to select Contact Omni from the Help menu within the app. Your message will be tracked and assigned a ticket number, and our support team will generally reply within one business day.
That said, I don’t believe OmniPlan has support for formatting just the title of a item, so perhaps that’s what you’re seeing. You should be able to change the font on an entire item, however. You can do this in the Styles Inspector, or by selecting the entire row before trying to change its font. (To select the row, click on the item’s leading bullet or in its left margin or press Command-Return.)
Hope this helps!