fix(images): use dark text on white background (#9863)

#9250 caused a regression by changing the 
background color of all images from white to the default background color, 
causing issues with images containing black elements on transparent background.

This reverts the image background color to white, but sets the text color to the 
default text color for the dark theme to resolve the original issue with alt texts.

Co-authored-by: Claas Augner <[email protected]>

Author: OnkarRuikar

Diff for: client/src/document/index.scss

@@ -83,9 +83,12 @@
   }
 
   img {
-    background: var(--background-primary);
+    /* In dark mode, required for images with black text on transparent background. */
+    background: #fff;
     border: 1px solid var(--border-primary) !important;
     border-radius: var(--elem-radius);
+    /* Required for alt texts. */
+    color: #1b1b1b;
     display: inline-block;
     display: flex;
     height: auto;