There appears to be no easy way to change the background color of stream item icons due to there not being a class around the span tag that wraps the icons based on the stream type. Right now stream icons use the classes "stream-icon pull-right ml-5".
When attempting to change the background, it does this for all of the stream icons which end up making all of the icons feel the same. So what I recommend is adding classes to the span tags based on their activity type. So for example for an article it could have the classes "stream-icon pull-right ml-5 es-article" which would not harm the display and allow us to change the styling. This would also allow for the icons to natively be styled through CSS instead of inline.