You can apply the following command both from the URL query string and config. The command applied in the URL query string always takes precedence over the same command present in config.
style= * cssPath *
Relative or absolute CSS location.
Specifies the location of the custom CSS file. If the cssPath is relative, it is resolved against the viewer HTML page location and the value of contentUrl= parameter.
All asset references within the CSS file are resolved against the CSS file location, not the location of the calling HTML page.