1) The single page for the testimonial is already created. You need to just create testimonials posts with title, content, and featured image from the dashboard. It will occur on the homepage. When the user clicks on the title it will redirect it to the single page where the user can read all the information.
2) You can remove sidebar widgets by following the steps below
Go To Dashboard >> Appearance >> Widgets >> Blog Sidebar.
3) Please provide us colour code and page URL where you want to add a background colour.