I want few pages to be accessed by guest profile to follow different styles and some for authenticated users. Header image to be varied for two different users. How can I achieve this? If I change the CSS on the branding it reflects the entire community but i want it to reflect only on the specific pages.
You can create a Custom Theme Layout which basically involves creating a lightning component, referenced here
You will then want to create a Page Variation which you can assign to different profiles referenced here Page variations allow you to specify what profiles access what page. From the community builder, select the page you want to create a variation and select the cogwheel (settings) that appears next to it: from there, you want to create the page variation and save it. This will allow you to select the audience criteria under the page settings:
Now, Once you created your Custom Layout (so pages are different from one to another based on profile) you want to go to each of your page variations and select page properties under each: and assign a Theme layout to each page variation (make sure the correct page variation is selected when you do this)
Repeat for each page that requires a variation and a different layout.