For media element, cssstyleselector loads the media controls style-sheet by using defaultTheme, which assumes the theme is needed in non-page dependent code. However, we may need to load a page dependent theme. For example, if the browser runs in a force-fullscreen-mode, (see https://bugs.webkit.org/show_bug.cgi?id=51133) then the theme of the media controls might be changed a bit. To change the theme, one possible solution is to load a different style sheet for the force-fullscreen-mode. Considering following code

String RenderThemeQt::extraMediaControlsStyleSheet()
{
    String result = String(mediaControlsQtUserAgentStyleSheet, sizeof(mediaControlsQtUserAgentStyleSheet));
    if (m_page && m_page->chrome()->requiresFullscreenForVideoPlayback())
        result += String(mediaControlsQtFullscreenUserAgentStyleSheet, sizeof(mediaControlsQtFullscreenUserAgentStyleSheet));
    return result;
}

To make above code work, we have to use themeForPage (which passes a page pointer to the RenderThemeQt) to retrieve extraMediaControlsStyleSheet.