SharePoint Online - Enabling the Client-Side Theming EngineSeptember 29, 2017
On Feb 24, 2016, Microsoft announced a new client-side theming engine for first release users. This new feature reduces the steps and time needed to apply custom themes. This speeds up development and introduces some unique use cases like client-defined theming or theming to match some dynamic event. Compare this with the old style theming engine where annotated CSS styles are compiled after using "Settings > Change the look" and remain static until the next manual theme change.