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Fix deprecation warnings

<pre class="EnlighterJSRAW" data-enlighter-group="" data-enlighter-highlight="" data-enlighter-language="generic" data-enlighter-linenumbers="" data-enlighter-lineoffset="" data-enlighter-theme="" data-enlighter-title="">DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($navbar-padding-vertical, 2)

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