There's a section on themes in the Device Encryption Administrators Guide, which should be in your DOCUMENTATION folder that you can follow to create your own themes. You can them deploy them as a fileset, making sure you remove the McAfee ones if they're selected.
Do keep in mind that such things are not supported from a support-point of view, a good thing to keep in mind is that if the files (eg. the imagefiles) become too large, then the preboot environment might have issues buffering the files, and result in a locking of your system..
As such first try to deploy such a modified themepack to a testmachine first and give it some reboots to see if there arent any issues prior to deploying it.
I manually edited the files to change the backgroun black. I couldn't remove the icons for the token types but I was able to remove the text by them by changing the text color to black in the INIs. It was a little tedious but I was able to get rid of all the faces and vibrant colors.
I have run into an issue when customising our login screen that I also noticed on the default theme. The "change password" text is missing the d and at the bottom right McAfee is missing the M. Any clue as to what would cause this, or how I could edit the themes to bring these letters back?
probably the locale.ini is messed up in your custom theme so it's using the wrong font.
try picking a language from the menu in pre-boot and see if they come back, if not then you've messed up the font section or locale.ini along the way somehow - re-apply the language pack and it will resolve unless graphics.ini is bad.