how to fix greyed out edit button

Layers based on KML files are most likely to cause such problems. KML does not offer all possibilities we use in QGIS, and has other features we can not handle in QGIS. In such cases, editing of the layer will be abandonned by greying the button out.

Best practice is always to load a KML, save as shapefile (or spatialite database), delete the KML layer and work on with the shapefile(s).

For the styling, you can save the styling for every layer you created as .qml file, and reload it in another project.