[NcAppSidebar] Close button not satisfies contrast for accessibility

Close button in app info panel is hard to see when screenshot is white or light coloured

Originally reported by @Justinzobel at https://github.com/nextcloud/server/issues/50045

Bug description

When opening the app info sidebar e.g. https://nextcloud.domain/settings/apps/enabled/whiteboard it is not possible to see the circular x button to close the sidebar as it is low contrast.

Steps to reproduce

1. Install/enable whiteboard app
2. Go to https://nextcloud.domain/settings/apps/enabled/whiteboard
3. Look at screenshot and try to find the close button

Expected behavior

Close button has higher visibility.

Suggestion: The x button has an on-hover circular background. Make this background always present and increase the contrast between the X image and the circle image so it is clear to the user where to click to close that sidebar.

Start by reproducing the issue in the NcAppSidebar app info panel at the whiteboard settings page and locate the close button component. Compare its appearance against a white or light screenshot, then adjust the close control so its circular background and X have higher visibility. Done means the button is clearly visible and remains easy to identify as the sidebar close action.