One of the most frustrating aspects of using free WordPress themes is that the one feature you would like is only available in the Pro version. Sydney (the theme that I use for this website) is an exception to that. Thank you Sydney Developers!
A particular favourite of mine is Astra. Its lightweight, well-written and an excellent starting point for a website. However, the stick header option is only available in the Pro version.
Well, I have found the perfect way out. Adding the following Additional CSS to the free Astra theme will give you the sticky header you were looking for for free!