Disable smooth scrolling feature

**Is your feature request related to a problem?**
I have my own scroll settings set in about:config, after installing the theme my scroll settings are overridden with the ones defined in the theme. My own scroll settings are gone and I need to set them again every time I open the browser.

**The solution I'd like**
Remove or allow disabling the included smooth scrolling settings, I don't think a UI fix should affect the behaviour of the browser or override user settings.

No source files or tests are named. Start by locating the theme rules that set smooth scrolling and check how they override Firefox about:config preferences; done means the theme no longer overrides user settings, or provides a documented way to disable those rules.

Written by the indexing model from the issue text.