If you want to stay anonymous, you need to think about every step when using your mail provider.
Registering your account
Accessing your mail account
Reading your mails
Writing your mails
You have to assume that your mail provider saves every step you do in your account. This seems especially true for GMail, but also it is also true for others.
So when you ...
You can't. If you want to stay anonymous, you cannot send an email through Gmail as you will lose your anonymity. One of the main things not to do while using Tor is to use Google services since your emails are not private and can easily be read and traced. No email service is completely anonymous though there are certain email services which are more ...
No, even if the user is using dark mode, light mode, or some other theme (system theme or app theme), this cannot be used to fingerprint the user if they are using the Tor browser. Detecting dark mode or light mode does not even work with the Tor browser. This does not affect the user's anonymity, privacy, or security in any way.
Normally you can use CSS, ...