I've been able to achieve the same result by using a user style sheet.
Create a text file in a safe place with the following contents:
body {
font-family: "Helvetica";
	font-size: 14px;
}
Let's say you named it mystyle.css
Then, in Safari go to Preferences... -> Advanced
Click on the Style sheet pop-up menu, and select Other...
Show Safari where mystyle.css is in the file dialog that opens.
You should see instant results.
kendall_john
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