Make the separator between apps in Appstore visible

In the appstore we render all apps below each other. There is `

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` element as a visual separator, but it is not that much visible, (on one of my monitors with lower brightness, it is actually invisible).
https://github.com/aiidalab/aiidalab-home/blob/0b1c618d72bf170b2a51192adcc6ca1727c2af02/home/app_store.py#L170, but (at least in my browser) it is not visible since its color is white.

We need to add some styling to it. We should also adjust the overall whitespace around the apps, it seems currently there's a bit too much of it.