As you said, this is custom, so it is unsupported and you will have to keep maintaining your modifications every time there is an update and hope that it does not break anything. Going with Eugene's suggestion is the ideal case, possibly adding some kind of non-blue background to the log to make it look pretty.
If that is not acceptable, and since you were already toying with the css, you should already know where to find the files that you need to modify.
In that case, load the login page in your browser and use the dev tools to find the location in the html where you want to put the changes (e.g. the div with "login-box"). Then simply add a centered image like so:
To have your changes applied, you will have to restart the Html5 gateway (e.g. by disabling it, applying the settings, enabling it again, and re-applying).
<img style="display:block; margin-left:auto; margin-right:auto; width:100px" src="path to image">