The configuration options you posted above are not the same as the ones used in the gallery whose URL you posted. The gallery on your web page uses the following configuration options.
gallerywidth="100%"
galleryheight="100%"
backgroundcolor="#ffffff"
textcolor="rgba(255,255,255,1)"
thumbframecolor="rgba(255,255,255,.5)"
showopenbutton="TRUE"
showexpandbutton="TRUE"
showthumbsbutton="TRUE"
usethumbdots="FALSE"
usefullscreenexpand="FALSE"
screenmode="SMALL"
imageshadowcolor="rgba(255,255,255,255.1)"
showautoplaybutton="TRUE"
shownavbuttons="TRUE"
The gallery on your web page sets screenmode="SMALL".
If you want to use the Back Button in Small Screen Mode, you will have to set showSmallBackButton="TRUE".
Also, please note that buttonBarBackColor (the background color for the Button Bar) is a Large Screen Mode configuration option only. (This is noted in the description for buttonBarBackColor on the configuration options page.) When you gallery is displayed in Small Screen Mode, the default value of rgba(0,0,0,.5) will be used for the buttonBarBackColor.
If you want to use a Spage Page but want your gallery to be displayed in Large Screen Mode wherever possible, set screenMode="AUTO" and showSplashPage="ALWAYS" (instead of screenMode="SMALL").