The yellow background is an indication that there are external participants among the members of that space, making it an important security feature. Go to the People tab to identify who is external to your organization. That feature enables everyone in the space to be conscious of what information and content should be shared there.
In some cases, those "participants" are really just bots, so you may be able to remove them, and drop the yellow background. Another option is to use Dark mode, where the external participants indication is a lot more subtle.
