Add changing Application Theme option in Appearance settings to README

[zHackro]

In the "Appearance" options in the settings (not under "Profiles"), you should also change the "Application Theme" option to the preferred theme flavor to make the top bar follow the theme, even though this is not listed in the README. The section containing "flavourTheme.json" is not very useful otherwise

[ruohki]

Man, you are a lifesaver, I was going crazy. :D

[sgoudham]

Happy to merge a PR containing updated instructions, I don't have access to a Windows machine at the moment to confirm the steps listed in the original issue.