SP2019 Modern site - Full page blackout

[NSDev2007]

Hi All, 

Is it possible to develop a solution in SPFx for SP2019 modern site that will blackout the entire page with just a message. 

Basic purpose of this to stop users of site from performing actions on the site until they follow few rules laid down by the organization. 

The old solution developed in .Net custom solution is working on the classic pages but not in the modern site. 

Thanks in advance for suggesting any other alternative way forward. 

[#2]

@NSDev2007  Found the solution. Achieved it using SPFx application customizer extension.

[#3]

@NSDev2007 

If you don't mind me asking, what's the purpose ?  and why not just add them to an AD group after they successfully completed those tasks and that way grant them access ?  Seems abit overkill to develop a custom app just to have it blackout