Custom CSS3 scrollbar styling can be used to customize the look and feel of scrollbars on a website. Here are 15+ different styles for customizing scrollbars using CSS3:

1. Classic Scrollbar: This style uses a simple, classic design for the scrollbar with a single color.
2. Gradient Scrollbar: This style uses a gradient effect for the scrollbar to add depth and texture.
3. Rounded Scrollbar: This style rounds the edges of the scrollbar to give it a softer, more modern look.
4. Shadow Scrollbar: This style adds a drop shadow effect to the scrollbar to give it a more three-dimensional look.
5. Flat Scrollbar: This style uses a flat design for the scrollbar to create a minimalist look.
6. Dark Scrollbar: This style uses a dark color for the scrollbar to make it stand out against a light background.
7. Light Scrollbar: This style uses a light color for the scrollbar to make it blend in with a light background.
8. Transparent Scrollbar: This style makes the scrollbar transparent to create a minimalist look.
9. Double Scrollbar: This style uses two scrollbars stacked on top of each other for a unique look.
10. Thin Scrollbar: This style uses a thin scrollbar to create a minimalist look.
11. Wide Scrollbar: This style uses a wide scrollbar to make it easier to use on touch screens.
12. Fancy Scrollbar: This style uses a fancy design for the scrollbar with decorative elements.
13. Neon Scrollbar: This style uses a neon color for the scrollbar to make it stand out.
14. Dotted Scrollbar: This style uses a dotted pattern for the scrollbar to create a unique look.
15. Striped Scrollbar: This style uses a striped pattern for the scrollbar to add texture and interest.