The technique for changing the icon will be similar to what was discussed in this post: https://community.adaptlearning.org/mod/forum/discuss.php?d=1713

You'll need to track down the .hbs template that governs the region in question. Plug-ins like adapt-contrib-accordion supply their own templates. However, some parts of the framework are "built-in". Look for more templates in src/core/templates. There you'll find navigation.hbs

Templates found within the theme will override core/templates, so always look for an overriding template there. In adapt-contrib-vanilla, you'll find them in the templates directory.

Vanilla uses the Open Sans font pulled from Google. Find that instruction in less/font.less. But it doesn't include anything that Vanilla is using as an icon. Without more information from you, I'd be sending you back to the fonts directory and less/src/icons.less as in the other post.