Sorry for the delay. Lots of stuff going on today, and this is something that may take a long time to describe.
I believe the control for whether your drawing styles library is Read/Write vs ReadOnly is within the Project settings. To access the Project settings, click on your File tab, then go down and click on Manage, then over and click on Projects. The 'active' project will be listed with a checkmark next to it. Those settings within the 'active' project will be ReadOnly if there are any Inventor documents open, so you may need to close all documents first, without closing Inventor. Then while that Projects dialog is open, and the active project is selected in the top list, there will be a property named "Use Style Library = ", that you may want to set to Read-Write, so that you will be able to make the needed changes. Then, if everything else there looks good, you can click Apply, Save, & Done buttons.
Now go to the Manage tab, and open the Styles Editor. The first items listed at the top of the list on the right are the 'standard' styles. Those are the top level styles, that hold all other types of styles, and all the styles that will be default while that standard is active. If you go to the Tools tab (outside of this dialog), and click on the Document Settings, you will see a drop-down control on one of the tabs for setting which 'standard' is the active one for the drawing. Within that 'standard' style is another type of style just for setting 'defaults'. You will see them on the left list also. One of those will be the one that the main 'standard' is using. While the main standard style is selected, if you look to the right, you will see a tab for setting which 'defaults' style it should use. And if you select that 'object defaults' style in the list on the left, you can control which individual styles will be used for which specific situations. And if you need to, you can create more of the individual types of styles, and set a different one as the 'default' within that 'object defaults' style that the active 'standard' style is using.
Wesley Crihfield

(Not an Autodesk Employee)