Update: Theme considerations added to README (fixes #65) (#104)

README.md

@@ -18,13 +18,24 @@ It provides visual accessibility improvements.
 
 ### Note
 * IE11 cannot apply filters. This means that images and videos will not be transformed in IE11.
-* All colour transformations are applied by mathematical shifts. It is therefore important that the course start from AA colour contrast for the algorithms to be applicable.
 * Invert only inverts brightness, not colour.
 * Line height, paragraph spacing, letter spacing and word spacing are all ratio based. 1 is the current value, 1.2 is and uplift by 20%, 0.9 would be a shift downwards by 10%.
 * In order to support paragraph spacing, all body text needs to be wrapped in [paragraph tags](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/p).
 * Font size medium is the default font size (16px usually), large is 18pt, small is 9pt.
 * Hide decorative images is contingent on alt text.
 
+### Theme considerations
+* All colour transformations are applied by mathematical shifts. It is therefore important that the course start from AA colour contrast for the algorithms to be applicable.
+* Any custom CSS `background-image` will need to explicitly define support for colour profiles. For example:
+<pre>
+html:not([data-color-profile=default]) {
+  .selector-with-css-bg-image {
+    .visua11y-filters;
+  }
+}
+</pre>
+* For any text that overlays a background image, ensure an appropriate `background-color` is set to provide sufficient contrast in the instance decorative images are hidden (`"_noBackgroundImages": false`).
+
 ----------------------------
 **Author / maintainer:** Adapt Core Team with [contributors](https://github.com/cgkineo/adapt-visua11y/graphs/contributors)<br/>
 **Accessibility support:** WAI AA<br/>