The default look of the toggle 'button' is defined within the CSS related sections of the documentation, in this particular case within the 10. ui-bar.css section. You can use the Web Developer Tools built into modern web-browsers to Inspect the HTML elements that make up the current page to determine which HTML element is involved, and the related CSS selector / rule that is being used to styling that element.
The HTML element you are looking for has an ID of ui-bar-toggle and the two CSS selectors / rules that you will need to alter are:
1. #ui-bar-toggle:before to remove the Character content being shown when sidebar is un-stowed.
2. #ui-bar.stowed #ui-bar-toggle:before to do the same for the stowed sidebar.
3. #ui-bar-toggle to add a background-image to the button.
4. [optional] #ui-bar.stowed #ui-bar-toggle if you want the stowed sidebar to show a different image.
The resulting CSS would start looking something like the following, and you would place it the the Story Stylesheet area.
#ui-bar-toggle:before, #ui-bar.stowed #ui-bar-toggle:before {
content: normal;
}
#ui-bar-toggle {
background-image: url("square.jpg");
}
/* Optional */
#ui-bar.stowed #ui-bar-toggle {
background-image: url("triangle.jpg");
}
note: obviously the image URL's in your copy of the above CSS would need to reference your image files.