Add auto darkmode (#33)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

_config.yml

@@ -20,6 +20,8 @@ plugins:
 # Theme-specific settings
 
 minima:
+  skin: auto
+
   # Minima date format.
   # Refer to https://shopify.github.io/liquid/filters/date/ if you want to customize this.
   #

_sass/minima/custom-styles.scss

@@ -0,0 +1,3 @@
+img {
+    background-color: $lm-background-color;
+}