Make input placeholder themable

app/javascript/styles/_variables.scss

@@ -97,6 +97,7 @@ $spacers: (
   --body-text: 0, 0, 0;
   --muted-text: 108, 117, 125;
   --input-text: 0, 0, 0;
+  --input-placeholder: 108, 117, 125;
 
   --turbolinks-progress-color: #a58adc; // --primary lightened by 25%
 }

app/javascript/styles/overrides/_inputs.scss

@@ -9,10 +9,14 @@
     border-color: var(--primary);
     box-shadow: .5px 0 0 0.1rem var(--primary);
   }
+
+  &::placeholder {
+    color: RGB(var(--input-placeholder));
+  }
 }
 
 .input-group-text {
   color: RGB(var(--body-text));
   background-color: var(--raised-accent);
   border: 0;
-}
+}