Can I change the layout of Safari?

There are no themes available for Safari.

But colors can be inverted.

This will produce a white on black effect as well as other color effects.

System Preferences > Accessibility > Display

Enable "Invert colors"

For details: https://support.apple.com/kb/PH25807?locale=en_US

Note:

This is a systemwide change.

You may not wish to have this systemwide change.

Disable it after testing.

Use a browser like Firefox, Chrome, or Opera that support custom visual themes — available through the browser interface.