I realise this is tricky because the scss serving happens differently from the user page request... (i.e. styles.php vs view.php)

But as well as the change in MDL-59148 one comment I made (and Eloy did too) is the idea is that we could notify the user (or at least admin) that outdated scss is being served so they realise why changing their theme setting hasn't caused colours to change.

A non-technical solution would be to just display a message any time you change theme settings that it might take some time to get applied. Another could be testing if the compiled files are on disk and generating a notification in view.php if not?