Internal issue 3048 - Dark mode issues

Stackblitz example: [https://stackblitz.com/edit/wn3wt6xa-qd55k4re?file=index.html|https://stackblitz.com/edit/wn3wt6xa-qd55k4re?file=index.html]

Despite explicitly setting the theme to light mode only using the {{ic-theme}} component, the text on the page is still changing from dark-mode to light-mode when that setting is changed.

This might be a case of “what should the behavior actually be?” rather than a bug, as it might be that developers not implementing dark mode should just fix these issues themselves for their particular applciation - however, if the intention was for dark mode to be an “opt-in” feature, this is not currently the case.

This is the kind of issue that might have evaded testing because it’ll only manifest under certain conditions - generally, the two big ones will be…

* A user explicitly sets a background color without explicitly setting a text color (they might not expect the text color to change when the user selects a dark mode preference, but it will!).
* A user explicitly opts out of supporting dark mode using the {{color-scheme: only light}} CSS style.

In my case, this resulted in a (mostly very easy) ICDS upgrade causing my app to become unusable if the user happened to have “dark mode” set as their preferred appearance (which I didn’t notice until after the app was in production).

Start with the linked StackBlitz example and its index.html, then trace how the ic-theme component applies theme styles and how color-scheme: only light is handled. Confirm the intended opt-in behavior with the issue discussion, and define done by reproducing the unwanted text-color change and verifying the agreed behavior in relevant theme tests.

Written by the indexing model from the issue text.