fix(a11y): control label text color

[huyenltnguyen]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

The ControlLabel component doesn't have text color set, so it has been using the default color (black), which makes the text unreadable in dark theme:

The issue can be seen here: https://opensource.freecodecamp.org/ui/?path=/story/components-formgroup--default&globals=backgrounds.value:!hex(0a0a23).

This PR adds a default text color to the component.

---

The root cause is actually a little deeper.

• ControlLabel doesn't have text color specified, so it falls back to the global body style
• body has color set to secondary-color

  • ui/src/global-element-styles.css

    Lines 383 to 388 in 5317d85

    	body {
    	background-color: var(--secondary-background);
    	height: 100%;
    	font-family: var(--font-family-sans-serif);
    	color: var(--secondary-color);
    	}

• But we don't have that token in our color list: https://github.com/freeCodeCamp/ui/blob/main/src/colors.css

We are removing global-element-styles.css as part of #61 so I don't bother updating the file.

Also, the issue luckily doesn't affect /learn. The element does need to fall back to the body text color, which is also secondary-color, but this token exists there: https://github.com/freeCodeCamp/freeCodeCamp/blob/50d7f03763e7d64690391bc7a004eccbdd75156d/client/src/components/layouts/variables.css#L43.

Screenshot

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