Hi there!
You can inspect element to find out if there is unique class exists in the offcanvas menu item already as follows:
https://prnt.sc/y2ca07
Then just need to use the unique clas to write down your own query to fit your need. You have to keep suct CSS code into the custom CSS field of the template.
Note: Path towards custom CSS:
Extensions>Templates>Styles>
Click on your current default template name then choose Template Options.
Then find custom CSS within the custom code.
https://prnt.sc/pu705g
https://prnt.sc/pu70zn
You can look into the blog too:
https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action
Hope this helps!
Best Regards