Open the document profile files....
Location may vary based upon operating system. Generally it's something like:
Applications/Adobe Illustrator/Support Files/New Document Profiles/en_US/ containing files such as...
- Art & Illustration.ai
- Film and Video.ai
Back up these files before making any changes
If you edit those files and then save them to the same location that will change the startup files for new documents. You can do things such as add custom Brushes, edit the Swatches, change the Symbols or Graphic styles, etc.
Simply edit, and
Save As, replacing the existing file (AFTER you've backed up the original files).
Then relaunch Illustrator. New documents should then reflect your changes.
Note that there is nothing which will edit existing files in the same manner. This only edits the "start" files for new documents.
Note that for versions prior to Illustrator CC, getting to the Document Profiles is much different. It can entail digging into the actual packaged application. Adobe didn't make the profiles so easily accessible prior to CC.