Set up a blank file using the style as you’ve edited it. Also make sure you’ve updated the style in the model. Then use File>Save as template to save it. Make sure the box for Use as default template is ticked.
When you open a new session of SketchUp it should give you your default style as the first option.
If you click on File>New to start a new file, it’ll open with that default template and thus the style you set.
This is all exactly the same as it’s been for at least two decades.