Changing 'background contents' setting in existing PS document?

In Photoshop Version 24 (year 2023) for Mac, the command you use to set the document background for a regular document (not an artboard) when you create it is found in the File > New window under the name Background Contents.  However, if you are working on a document and do not like the background you chose, you can change it for that document (on Mac) in Apple icon > Settings > Transparency & Gamut (I believe the setting command is named Preferences in Windows.) 

Here, you can change both the size of the boxes in the transparency grid and their color, or choose a solid color in place of the transparency grid.

Tip:  White backgrounds on my display screen bother my eyes, and I just discovered that changing these settings to the ones shown above knock down the brightness of the transparent canvas noticeably without making it harder to see the contents of Layers panel thumbnails.

Note:  If you only want to change the background contents for one file, you will need to go back and change the default in Settings (Preferences) at some point.

With newer versions of PSD (2020, for sure), select your artboard using the Artboard tool.  Up in the top bar — right side of Size, Width, Height —is a dropdown from which you can select Transparent, White, Black, or Other…; that's your background canvas setting for the document.

Selecting the Background Layer, setting Foreground Color and filling (or changing the Background Layer to a regular one and removing it) should work out fine in an Action.

Hi. If the background is a separate layer just fill that layer with the color or erase it the pixels for getting transparency. Otherwise, you need first manually select the background.