Update header style on about:preferences pages

[bsclifton]

Test plan

#8237 (comment)

1. Open about:preferences#search
2. Make sure there is margin under the top header

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When reviewing the Extensions page, I noticed that the header style didn't match the other pages:

The orange style looks great though- we should consider putting this style on the top of each page (possibly with a description of what the page has).

Here's the page where I noticed the new style (which looks great)

cc: @bradleyrichter @cezaraugusto @luixxiul

[luixxiul]

IMO creating custom elements as I did for commonForm would solve the inconsistency issue like this.

[cezaraugusto]

@luixxiul mind grabbing this?

[luixxiul]

@cezaraugusto no problem, I'm taking it.

[luixxiul]

To change the "extensions" header into the orange one, we would at first need to add the same type of header to other tabs such as about:preferences#searchand about:preferences#plugins. Otherwise it would lead additional style inconsistency.