I personally think that if you want a background, you should be able to change background each slide. At the moment if you choose a background at the moment it is portrayed to every other slide that you have added to your presentation. However, if you do not want this you can use the format background option under the design section, but even so you cannot change the cool design of a slide very much. I think that you should be able to change the design and make it so that you can easily and effectively change the look of each individual slide.
You can set a background in PowerPoint Online by going to Design>Format Background. Thank you for using PowerPoint!