Support
Right Battle, a free Chrome extension. No account.
The line lives in this browser only. There is nothing to sign in to, and nothing to recover from a server.
Change the line
Open a new tab and use the gear. Edit each line on its own. Add a line, or remove one, from that same panel.
The filled rainbow dot is the line you want in color. An empty ring stays black on the light page, and stone on the dark one. Ink and Cinnabar paint every line the same color.
Font, color, paper or night, and size are in that panel too. They stay on this device.
If the new tab did not change
Another extension is probably taking that page. Open chrome://extensions, turn the other new-tab extension off, and leave Right Battle on. Then open a new tab again.
If Right Battle itself is off, turn it on. Chrome only allows one extension to own the new tab.
If the line disappears
Removing the extension, or clearing site data for it, deletes the line. There is no backup. Write it again from the gear.
Contact
Questions go to the contact email on the Chrome Web Store listing for Right Battle. What the extension stores is described on the privacy policy.