I have a similar question as @drnextgis in this post: I made a change to the symbol colour of one value out of four categorised values and aim to automatically apply it to other layers in the same QGIS project using the same symbology of the same .qml.
I'm working in one QGIS project with about 40 layers (landparcels, 1.8 GB in total) that all have the same attributes and therefore share the same
.qml-file is set as default for each layer. If I change the symbol colour of a value in one of the 40 layers and select 'Save Style...' and then overwrite the existing .qml, the change only applies to this particular layer. It neither makes a difference if I select 'Save as Default'.
I use the LTR version of QGIS (3.10.9.).