i got the same problem/idea 2 days ago and made a post about and got a response which i hadn't made to work yet (not the time to test) so i link it here hoping this might help someone
The answer by JR
It's possible to use data defined style and variable in combination to achieve that.
Basically you create project variable like "Outline_variable" and give it one of the accepted value :
(Expected input:string [no|solid|dash|dot|dash dot|dash dot dot])
Then you set the data defined symbology to variable>Outline_variable for each of you layer
Now you just have to modify the project variable from 'solid' to 'dot' to change the outline of all your layer (and of course you could set as many variable as you need for controlling symbology parameter like stroke colors, stroke width, fill color, ....)