Go here:
https://github.com/RetroPie/RetroPie-Setup/blob/master/scriptmodules/supplementary/esthemes.sh
Click this "three dots" and choose "edit file":
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(It's the second three dots, below the upper three dots.)
It will show you this message or something like it (yours might be worded slightly different, since you don't yet have a fork so you'll be creating it in this step):
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Add your theme to the list here. "YourName" is the name of your GitHub account that hosts the theme and theme_name is the theme name that will be displayed in the theme listing. The repo should be called es-theme-theme_name and when you put it here, leave off the es-theme- part and just write 'YourName theme_name'.
Click "propose changes":
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...this creates your own fork, with the change that you made. Also, it automatically brings you on this page for "comparing changes"; click "create pull request" once to begin the process of creating a pull request:
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Here you can fill in some information about the change, and then after reviewing everything is correct, click "create pull request" a second time to finalize and submit the pull-request, where it can be reviewed by the developers:
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