The secondary toolbar CSS rules predate the general availability of CSS
nesting, which makes them more difficult to understand and change
safely. The primary issues are that the rules are spread over the
`viewer.css` file, they share blocks with other elements and the scope
of the rules is sometimes bigger than necessary.
This refactoring groups all CSS rules for the secondary toolbar button
container/icons together, scoped to the top-level `#secondaryToolbar`
element, for improved overview and isolation. Note that this patch only
intends to move the existing rules around and not change any behavior.
Moreover, this patch does not move the rules for the secondary toolbar
itself and the secondary toolbar buttons; those will be part of a
follow-up patch and will be easier once this is in place first.
Co-authored-by: Calixte Denizet <calixte.denizet@gmail.com>
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