You can see the edit active theme on your local host because that is where you are developing your app and making design choices. This option is removed when you deploy because that means that your app is live on the internet for anyone to see. At that stage, you won’t want any random people to able to go into the settings of your app and completely change the custom theme you made. Thus, when you deploy, the viewer of your app can only choose between the theme you made for them, and the two Streamlit default themes (light and dark).
But, to edit the active theme on a deployed app as the developer of that app is super easy. Streamlit’s Sharing platform is “intelligent” and is able to detect changes in your GitHub repo. When it detects a change it automatically updates your app to have the newest commit from your main branch (or whichever branch you deployed from)
That means, you can continue to refine your colouring theme locally, then commit those changes and you app will update with your new color selections!