I have managed to find a solution:
Go to http://tmtheme-editor.herokuapp.com (someone has built a web-based theme editor). Once you have tweaked the colour syntax you can download the themename.tmTheme file. Move that file to /Users/username/Library/Application Support/Sublime Text 3/Packages/ User/ (if you are on a Mac).
Load in the syntax theme from the top menu: Sublime Text > Preferences > Color Scheme > themename.
If you want to tweak you syntax further up can upload the custom theme back in the web-based theme editor and save it out again.
Hope this helps someone else.