Hide diff color user preferences by default to reduce visual weight

The following discussion from !78281 (merged) should be addressed:

• @pedroms started a discussion: (+1 comment)

  @pedroms: @wwwjon 🤔 suggestion (behavior,non-blocking): the new settings increase the size substantially and, I assume, will only be used by a small amount of users. So I'm wondering if we can make these new settings behave more like an “advanced” section, that appears only when the user decides to customize the colors. Here's a rough example:

  Unchecked (default)	Checked

  Again, this is non-blocking, so I'm okay with tackling this separately from this MR. I'd love to hear your thoughts nonetheless!

  @wwwjon: I don't have a strong opinion on that. But I guess that a normal user probably visits the settings page very rarely. And with your suggestion, the setting will be very much hidden, which will not help anyone. By the way, the preview is very helpful for choosing the syntax highlighting theme! So at the moment, I would refrain from this suggestion.