how to create a Fluent UI custom theme for Blazor

So I'm looking at the design theme documentation https://www.fluentui-blazor.net/DesignTheme

It looks like in my App.razor I am meant to create my custom theme like so:

<FluentDesignTheme CustomColor="#00FFFF" StorageName="customtheme" />

Which is meant to add a record to my localstorage like so:

"customtheme" = {"primaryColor": "#00FFFF"}

but this theme does not appear in my local storage.

How can I create a custom theme and apply it to a component on demand? I don't want to change my entire app, but even if I did I couldn't get the documented examples to run in my v4.3.1 project. Any help would be greatly appreciated! Maybe someone has a sample repo I can look at for theming? After reading the documentation I'm not sure the Fluent UI theme system works or will work for my needs (creating a theme and then applying it to a component)

Thanks!