When editing a custom theme, a new window with the theme editor opens, listing all the files available for edit. To find the content you want to edit, you can either search for the name of the file or a part of the text you'd like to modify.
To identify the name of the file, right-click the content 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 -->.
For instance, to edit the default "Payouts" paragraph in the mini-site signup page, you can search either for the "signup_form.tpl" theme file or a portion of the paragraph's text, e.g. "Minimum payout is $100".