Hi Kelvin

It is possible to create your own login instructions using the text editor and include the images and content you want.

This can be found under Site administration > Plugins > Authentication > Manage authentication  Instructions

I am not sure what you are meaning by two themes - it is possible to force a theme in a URL by adding  ?theme=themename to the Totara page .

Site administration > Appearance > Themes > Theme settings - Allow theme changes in URL

Regards