FIX: Don't filter images that support the dark theme (#942)

Author: QuLogic

docs/user_guide/light-dark.rst

@@ -103,8 +103,14 @@ Change the theme and a new image should be displayed.
 Images and content that work in both themes
 -------------------------------------------
 
-For images without the :code:`only-dark` class, when the **dark theme** is activated, a white background will automatically be added to ensure the image contents are visible.
-If your image is suitable for both light and dark theme, add the CSS class :code:`dark-light` to make your image theme-agnostic.
+When the **dark theme** is activated, images that do not support dark mode will
+automatically have a white background added to ensure the image contents are
+visible, and their brightness will be reduced by a filter.
+
+If your image is suitable for the dark theme, add the CSS class
+:code:`only-dark` as noted above. If your image is suitable for both light and
+dark theme, add the CSS class :code:`dark-light` to make your image
+theme-agnostic.
 
 For example, here's an image without adding this helper class.
 Change to the dark theme and a grey background will be present.

src/pydata_sphinx_theme/assets/styles/variables/_color.scss

@@ -103,8 +103,11 @@ $pst-semantic-colors: (
       .only-light {
         display: none !important;
       }
-      // adjust images in dark mode
-      img {
+      /* Adjust images in dark mode (unless they have class .only-dark or
+       * .dark-light, in which case assume they're already optimized for dark
+       * mode).
+       */
+      img:not(.only-dark):not(.dark-light) {
         filter: brightness(0.8) contrast(1.2);
       }
       /* Give images a light background in dark mode in case they have