APEX 5.1
Universal Theme
Oracle 12.1
Chrome Version 57.0.2987.133 (64-bit)
Firefox 53.0
IE 11.1066.14393
I have a problem in that the logon page is rendering on only half the browser page in Firefox and IE but renders across the full page in Chrome for exactly the same code.
Note that the developer toolbar spreads across the whole page for all browsers.
e.g.
CHROME : )

FIREFOX and IE : (

The only code I've added is the following CSS in the page Inline CSS for the background image.
.t-PageBody--login .t-Body {
background: url(/i/VXVIEW/images/telco_bground.jpg);
background-size: contain;
background-repeat: no-repeat fixed center ;
}
If I remove the CSS code, the background image does not display but the logon popup is still left justified and not centered. What CSS code would be affecting this and is not recognized by IE and Firefox?
Anyone have any ideas where to look for a fix?
thanks in advance
PaulP