With your graphic styles window open, create a line on your artboard with the settings you'd like. With the line not selected, hold alt/option, click and drag the line onto the default graphic styles thumbnail. I have this setting established in a specific workspace, so not sure how to do so as a permanent setting. Hope this helps!
With your graphic styles window open, create a line on your artboard with the settings you'd like. With the line not selected, hold alt/option, click and drag the line onto the default graphic styles thumbnail. I have this setting established in a specific workspace, so not sure how to do so as a permanent setting. Hope this helps!