@Nonnull
    @Override
    public String resolveThemeName(final HttpServletRequest request) {
        val theme = request.getHeader(this.themeHeaderName);
        return StringUtils.defaultIfBlank(theme, getDefaultThemeName());
    }
This is {@link RequestHeaderThemeResolver}.

@author Misagh Moayyed
@since 5.2.0
support/cas-server-support-themes-core/src/main/java/org/apereo/cas/services/web/RequestHeaderThemeResolver.java