OK, with the votes as they are and a lack of verbal support for the option, I'll call it as it is. There won't be any option to change the background colour, either globally or per-theme (note that it's actually the style's theme which determines the background colour).
Whilst I can see the potential benefit of a more dedicated option (such as one that only affects the OG themes), it's not really worth the effort for 2 reasons:
1) It's still technically possible for a level designer to use the terrain and objects from one of these styles, but set a totally different theme with a different background colour. And vice versa. The only way to catch all possible cases is to implement the option globally, which I think we can all agree is too heavy-handed a way of dealing with this.
2) It's very quick and easy to just manually change the colours for the OG styles' themes (and indeed any style's theme you wish) anyway. So, if there's a particular theme which has a background colour that you don't like, you can set it manually, and then any level using that theme will have its background colour changed to your liking. Here's a tutorial, in case anyone doesn't know how to do it:
1) Navigate to the styles folder in your copy of NeoLemmix or SuperLemmix.
2) Navigate to the style folder for which you wish to change the background colour.
3) Open the folder, and then open the theme.nxtm file in a text editor.
4) Under $COLORS, you will see BACKGROUND and a hex value (e.g. x000000) - this determines the background colour for the style's theme. Any level using this theme will have that background colour.
5) There's a handy website here (https://www.htmlcsscolor.com/#wheel) which can generate a hex value for any colour. Choose your desired colour and make a note of its hex code.
6) Change the hex value of the BACKGROUND in theme.nxtm to your desired colour.
7) Done!
Reminder: Even if you change these values, it will still be possible for levels to be created using the terrain and objects from a particular style, but with a totally different theme determining the background colour. The way to solve this is find out which style the level is using as its theme, and then change that theme's background colour.
(If you wish to change all themes that have a particular background colour, you can change the value of multiple themes simultaneously using Notepad++, I will make a separate tutorial for that soon).