I am not sure if this is an error in the ‘Standard’ and ‘Standard Dark’ theme but the property ‘--rz-layout-background-color’ is set to ‘transparent’. All the other free themes are set to ‘var(--rz-body-background-color)’.
This causes the body background color to be black when selecting the ‘Standard’ theme for me using a dropdown. I am not sure how or why it works on the Radzen site.
As you can see the background around this component:
This is intentional. The Standard theme uses a transparent --rz-layout-background-color because the <body> itself has a background image within the theme.
If you change --rz-layout-background-color to var(--rz-body-background-color), the layout will cover that background and the Standard theme will no longer have its intended appearance.
You can see how the Standard theme is supposed to look here:
If the background is black in your application, I would check whether you have any global CSS overriding the <body> background when switching themes.
I think this is a theme issue rather than a problem with your component. In the Standard and Standard Dark themes, --rz-layout-background-color is set to transparent, while the other themes use var(--rz-body-background-color).
When switching themes through the dropdown, the layout therefore loses the expected background and exposes the body background underneath. Setting the property to var(--rz-body-background-color) fixes the issue for me. It would be worth checking whether the Standard themes are missing the same variable used by the other free themes.