💄 style: don't fully invert images in dark theme

It washes out the contrast a bit, but a pure white background on a
picture, which matches the background of the light theme, will now
perfectly match the dark theme background.

sass/main.scss

@@ -61,7 +61,7 @@
   --accent-color: #ff9a8c;
 
   .invertable-image {
-    filter: invert(1);
+    filter: invert(.88);
   }
 }