During the process of customizing a theme, the theme editor lists all the files available for edit. To find the content you want to edit, you can simply search for a part of the text you'd like to modify. For instance, to edit the default "Payouts" paragraph in the mini-site signup page, you can search for "Minimum payout is $100".
If searching for a part of the text is not applicable, alternatively you can search by the name of the file. To identify the name of the file you need to edit, right-click the element you wish to change and select "Inspect" to open your browser's dev tools. Then, look for the nearest HTML comment above the highlighted code - the text enclosed in <!-- -->, e.g. <!-- THIS IS A COMMENT -->.
As an example, to edit the Facebook page link in the footer of your mini-site, search for "footer" and access the "footer.stpl" theme file.