Why is my theme not applied?

I have a fresh installation of Grav, with the admin interface. I went to themes, and I added a new one, specifically Agency v1.4.0. It installed fine, and I set it as the active theme:
<img src="//cdck-file-uploads-canada1.s3.dualstack.ca-central-1.amazonaws.com/flex036/uploads/getgrav/original/1X/c354cae2a2dfb2942bc0e9ca994b6537bb2f0a27.jpg" width="283" height="500">
I then made a new test page, and it is set with the default theme, which I assume is the active theme:
<img src="//cdck-file-uploads-canada1.s3.dualstack.ca-central-1.amazonaws.com/flex036/uploads/getgrav/original/1X/7a6806b878c1dd1dc9937c465196957889058e5a.jpg" width="333" height="354">
However, when I create a test page, it looks like there is no theme at all:
<img src="//cdck-file-uploads-canada1.s3.dualstack.ca-central-1.amazonaws.com/flex036/uploads/getgrav/original/1X/50ba251f7b318c5c2e811162e27b11194e21c44f.jpg" width="555" height="435">
I'd like to not only apply the theme, but also start out with the nice layout and graphics that I see in the theme browser, so that I have a page less blank to start experimenting with. Why does it look like I have no theme installed after downloading it and making it active, and how do I get a page to look like the example in the theme browser?

Hello,

I believe that your theme is indeed installed.
If you want to start with the pictures and some content, I would suggest you to download the skeleton instead of the theme.
You can find skeletons here :
https://getgrav.org/downloads/skeletons

This way, you will already have some content.

The theme is correctly applied. The page seems unstyled because that's how the Agency theme styles a page of type "default".

The Agency theme demo in particular is built for a one-page site, the demo https://demo.getgrav.org/agency-skeleton/ for example is built using this page structure: https://github.com/getgrav/grav-skeleton-agency-site/tree/develop/pages/01.home downloading the skeleton will make your site look exactly like that, and you can start from there, as @AmauryH suggested.

Thank you for your responses. I was unclear on the difference between a theme and a skeleton, and I think I understand now.

However, I'm still unclear on how a skeleton will interact with the Grav-admin installation I currently have installed. It seems that if I use the skeleton, it is separate from the Admin interface. How do I keep the admin interface features when implementing a skeleton?

Thanks for responding.

I think I understand the situation now. I thought the Admin interface was more general and could handle multiple Grav websites, but now I see that it is more limited than that.

Thanks everyone for the explanations and help!