Make StyleDef member variables private.

[rpatters1]

Resolves: #30919

The data members of StyleDef appear to have been inadvertently made public. This PR corrects that.

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[cbjeukendrup]

The plan was to merge this via #31006, but I hadn't noticed that you had rebased this two days ago, so my merge commit only included an old version of this PR so the trick didn't work. So I'll have to close this as "kind of manually merged".