I just discovered this post and I found what I searched here. But I was wondering if there was a way to have multiple layers (let's say l1, l2, l3, l4) being updated automatically when you change something on one of them.
For example if I change the outline style of l1 from solid to dot I would like to see l2, l3 and l4 updated the same way without copy/pasting the style like mentioned in the link
(I'm using qgis 2.18.28 on windows)
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, ....)
