To add on to what @joshua wrote:
You can't host assets in FusionAuth (like CSS or images). You must host those externally. This post might be useful to you: https://fusionauth.io/community/forum/topic/1306/parameterizing-themes To offer a localization drop down, you need to build the select box in your theme or you can use the localSelector helper macro. This will iterate the supported locales for a theme and build the dropdown. Then, when it is changed, the page will forward to the same URL with a new locale parameter, which should render the page properly. More on the localization of themes here: https://fusionauth.io/docs/v1/tech/themes/localization I'm not familiar with Material UI but if it is javascript, html and css, it should be workable.