Form <fieldset> and <legend> question

I want to split a form in several sections. When I use <fieldset> and <legend> tags it works, but it doesn't look like it belongs to XF.



At the same time I can see that XF has form sections that look harmonic, for example Account Details.




Is there some classes that I should use? Or I should use some specific XF tags? I can't figure it out.
Please advice.

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