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