Add consistent checking of first dark then light asset

Author: viktorrenkema

packages/gitbook/src/components/PageBody/useCoverPosition.ts

@@ -37,8 +37,8 @@ export function useCoverPosition(imgs: Images, y: number) {
 
     // Load original image dimensions if not provided in `imgs`
     useLayoutEffect(() => {
-        // Check if we have dimensions from either light or dark image
-        const hasDimensions = imgs.light.size || imgs.dark?.size;
+        // Check if we have dimensions from dark (if provided) or else the default light.
+        const hasDimensions = imgs.dark?.size || imgs.light.size;
 
         if (hasDimensions) {
             return; // Already have dimensions