How can I allow using .svg files for the logo field, in the theme settings?
The logo field in the theme settings says it accepts
.svg format but it actually does not. As per the error message that comes up only
png jpeg jpg jpe gif extensions are allowed at the moment. Its a bug that is being worked on here. Try any of the patches mentioned in the issue and report there if your issue is resolved.
Alternativelly for your theme (or custom theme) you can just put the logo.svg in the theme folder and modify
page.html.twig to use the .svg extension.
I think you're looking for this: Unnecessary restrictions on logo format: Can't upload replacement SVG logo
You can find some patches that will enable this functionality.