Since we are able to install plugins from Firefox/Chrome we should also be able to install themes. Although you can install them, they don't do anything. Probably the themes are not as easy to implement because Safari looks different than those browsers however then there should be themes under Orion's own preferences and the ability to let users make their own themes and upload it.
Safari is the only browser that I know that doesn't use favicons next to the bookmarks on the bookmarks bar. This is standard practice with every other browser and I feel it should be here as well. It should be offered at least as an option to turn on/off. Having a long list of website names in the bookmarks bar on dark mode is a horrible reading experience.