Open the old SketchUp file in SketchUp.
Open the Styles browser.
Open the secondary pane. (little plus icon in upper right corner of Styles browser.)
Set one pane to In Model. (click on the house icon)
Set the the other pane to an existing library or make a new one.
Click on and drag the thumbnail for the style from In Model to the other pane.
That will save your style for future use.
Now, to share the style, navigate to the folder in which you saved the style and you should see it to attach it to a post here.
As to your second question, you can open a style in Style Builder and make modifications to it as desired.