For Android, the Opera browser comes with built-in Dark mode,
To Opt dark more, click the profile Icon at the bottom right corner and there you can see the darkmode, just toggle it and everything becomes black.
And why is the editor text so big and got underlines all over it? I must have been using my iPhone for too long. Lol
Underlines are probably as a result of spelling errs, so you might wanna check the spelling or it's probably as a result of differences in US and UK English versions.