How To Enable Background Support
Whilst the Default FM20 skins natively support custom backgrounds, they don't have an overlay layer so you may find your custom backgrounds leave bits of the game unreadable, so with my Mint Skins I have re-enabled the background overlay and included several files of different transparency levels so you can adjust the strength of the overlay yourself as a suitable level comes down to personal preference.
To Adjust the Overlay Strength
After installing the skin if you browse to the graphics\boxes\custom\background folder for the version of the Skin you are using you'll notice several graphic files these are numbered by the level of opacity - 0 is fully transparent and 100 is fully solid.
For these skins I haven't included a default paper file so they will use the default one from the game which is fully transparent.
To adjust the strength of the overlay, select the graphic with the level of transparency you want and rename it to paper, next make a copy of this file and then rename your copy to paper@2x. The @2x file is used for the zoom in mode.
To get the game to read your changes make sure the skin cache is off and reload your skin and the changes should apply.
To undo your changes or to pick another option if you first one isn't to your liking, first delete the paper@2x file then rename the paper file back to its existing number, then follow the previous instructions to rename a different file, then it is just a case of trying the different options until you find a level that suits you.
To Change the Overlay Colour
Inside the boxes\custom\background folder you'll also notice an xml file called paper, this file controls the colour of the overlay.
To recolour the overlay open the xml file to paper locate this line:
<colour id="red_replacement" name="bg"/>
The name bit determines the colour of the overlay, to change the colour change this value to a different colour name which you can get from the settings file for the skin you are using.