fix(widget): Slider text color should be light on dark background

Recent changes to Hero widget added support for both light and dark
options for background and text (previously it only supported dark
backgrounds and light text).

Slider continues to only support dark style (dark bg and light text).
However, since it inherits some styling from Hero, it began displaying
dark background and dark text. This fix re-applies dark mode to the
Slider widget.

layouts/partials/widgets/slider.html

@@ -13,7 +13,7 @@
   <!-- Carousel slides wrapper -->
   <div class="carousel-inner" style="min-height: {{$page.Params.height | default "300px"}};">
     {{ range $index, $item := $page.Params.item }}
-    <div class="wg-hero carousel-item{{ if eq $index 0 }} active{{ end }}" style="min-height: inherit;
+    <div class="wg-hero dark carousel-item{{ if eq $index 0 }} active{{ end }}" style="min-height: inherit;
       {{ if $item.overlay_color }}
       background-color: {{ $item.overlay_color | default "transparent" }};
       {{ end }}