If you download a browser extension called Stylus for Firefox and Chromium browsers, you can override the CSS on other websites.
I don’t use Safari much and haven’t tried it, but there is a setting to add a custom stylesheet under Advanced → Style Sheet.
I think this CSS will work:
.d-editor-input {
font-family: var(--font-family) !important;
}
(Replace var(--font-family)
with another font name if you want a different font.)