How to change the background color

I am not able to change the background color in the bloom

Sorry, can you say a bit more about where you want to change the background color? For example, do you want to change it for a particular picture, the cover, a block of text, all pages, or something else?
Are you trying a particular command and it is not working, or can’t you find a command to do what you want?
Bloom does not currently offer a lot of support for background colors, but some things can be done with custom CSS files.