Configure branded logos for dark mode

(I have now enabled GHA to automatically compile the css if changed,
this may make rebasing from upstream a weird affair)

Author: Robadob

inst/pkgdown/templates/header.html

@@ -19,7 +19,8 @@
     <div class="col-md-8">
       <div class="large-logo">
         <a href="https://www.sheffield.ac.uk">
-          <img alt="University of Sheffield" src="{{#site}}{{root}}{{/site}}assets/images/UOSLogo_Primary_Violet_RGB.svg">
+          <img alt="University of Sheffield" src="{{#site}}{{root}}{{/site}}assets/images/UOSLogo_Primary_Violet_RGB.svg" class="light-only">
+          <img alt="University of Sheffield" src="{{#site}}{{root}}{{/site}}assets/images/UOSLogo_Primary_White_RGB-dark.svg" class="dark-only">
         </a>
         {{#yaml}}{{#life_cycle}}
         {{#pre_alpha}}

…ts/images/UOSLogo_Primary_Violet_RGB.svg …ce/images/UOSLogo_Primary_Violet_RGB.svginst/pkgdown/assets/assets/images/UOSLogo_Primary_Violet_RGB.svg renamed to source/images/UOSLogo_Primary_Violet_RGB.svg inst/pkgdown/assets/assets/images/UOSLogo_Primary_Violet_RGB.svg renamed to source/images/UOSLogo_Primary_Violet_RGB.svg

@@ -88,7 +88,7 @@ $color-mode-type: data;
       color: #fff;
     }
   }
-
+  
   svg .icon {
     stroke: var(--bs-body-color);
   }
@@ -731,3 +731,13 @@ div.tab-content {
 }
 
 }
+
+// A few UoS specific dark/light theme rules
+// Hide the white university of sheffield logo during light mode
+[data-bs-theme=light] img.dark-only {
+  display: none;
+}
+// Hide the purple university of sheffield logo during dark mode
+[data-bs-theme=dark] img.light-only {
+  display: none;
+}