Defining some terms
"Extension" will be used to refer to Dark Mode for Canvas, the web extension available on Chromium and Firefox browsers
"Personal Information" will be used to refer to any information that can be directly used to determine the true identity of a user. Includes (but is not limited to) information such as name, email address, or phone number.
"Generic Information" will be used to refer to any information that cannot be directly used to determine the true identity of a user. Includes (but is not limited to) computer operating system, Extension version number, or whether the Extension is enabled/disabled.
Does the Extension collect or store Personal Information?
Nope. The Extension does not collect, use, or store Personal Information in any way. The Extension does NOT contain any pesonally targeted or non-personally targeted ads and is completely open source. If you have privacy concerns, check the source code on Github.
Does the Extension collect or store Generic Information?
Technically no. The Extension itself does not collect this information, but Google does collect this information and stores it in the Chrome Web Store Developer Dashboard. This Dashboard allows Marissa Rousseau, the extension's primary developer, to view Generic Information regarding the status of the Extension installed on the user's device, such as the Extension version number or Extension enabled/disabled status. Firefox Addons works in a nearly identical fashion to the Web Store Developer Dashboard.
What permissions does the Extension need?
The Extension uses a thing called Content Scripts to provide dark themes to certain websites. Do a quick Google search (or DuckDuckGo search for privacy's sake) to learn more about how Content Scripts work. That being said, the Extension cannot collect or store usernames nor passwords or any other form of Personal Information.