How can we change the css of the chatter feed for my visualforce page? Should we create the complete feed separately or is there any shorter and efficient way for this?
To short answer this "There is no shortcut"
If you are trying to restyle you will have to roll your own chatter(Using Connect in apex and custom HTML and CSS and JS) .This is not recommended approach since chatter comes with lot of features and replicating the same means involves lot of custom code and investment on code maintainence
If you are looking for rolling out chatter with SLDS then there is a BETA tag in lightning which can be used via lightning out