[css-flexbox] Example 4 (for collapsed flex items) is unreadable in dark mode

[dholbert]

STEPS TO REPRODUCE:
(1) Set your OS to use a dark theme (to trigger the dark-mode rendering for CSSWG drafts)
(2) View Example 4 in the flexbox ED, roughly here:
https://drafts.csswg.org/css-flexbox-1/#collapsed-flex-item

ACTUAL RESULTS:
The example has ~unreadable light-gray text on a white background. Screenshot:

EXPECTED RESULTS:
Readable text in the example.

[dholbert]

It looks like this is because:

• there's a global dark-mode switch that changes color to the light-gray color
• the example manually sets background: white for this section, but does not set color, so it's at the mercy of its inherited color to determine whether the text is readable over its white background or not.

The example probably wants to set color: black alongside its background:white, I presume. (Or it wants to be rewritten to avoid using background at all.)