Install Themes preview missing background color

[mor10]

As seen in the attachment, the preview function under Install Themes is missing the background color causing the info text and theme preview to be superimposed on top of the Install Theme page. The behavior is consistent for the theme info pane on the left and the transparency of the background in the theme preview itself seems related to whether the previewed theme has a color defined in the body tag. For reference the example grab theme does not have this color so it is entirely transparent.

[mor10]

Install Theme preview transparency

[dd32]

In 26317:

Theme Installer: Fix the Theme Preview function, and bring the button handling/styling inline with the Customizer. Fixes #26129