Change default font in Safari
Is there still no easy way to change the default font for Safari running on macOS?
Apparently there wasn't as of 2020:
Change default font in Safari - Apple Community
Mac mini, macOS 15.3
You may want to use this link to send Feedback:
The Feedback Assistant is intended for pre-release software
.Intro to Feedback Assistant on Mac - Apple Support
Safari displays the website as intended by the web designer who specifies the exact font to use when displaying the page. Overriding is still possible by using Style Sheets in the Advanced section of Safari, but it is not as clear as it is with other browsers such as Firefox that has a setting to choose the font to override what the webpage has specified to be used .