bug(web): when dark mode is on the label on spacebar is not clearly visible

**Describe the bug**

This issue was found when testing this PR: https://github.com/keymanapp/keyman/pull/5555.

On any web browser on macOS 11.5, if the dark mode is turned on the label on the spacebar is not clearly visible. See the screenshots below.

Dark mode on when not press and hold the spacebar:

Dark mode on when press and hold the spacebar:

Dark mode off:

**Expected behavior**

The label should be clearly formatted so that it is visible in both normal and dark modes.

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**KeymanWeb:**
- OS: macOS Big Sur
- Browser: Chrome 92.0.4515.131 (x86_64), Firefox 89.0.2 (64-bit), Safari 14.1.2
- KeymanWeb Version: of #5555 and master branch
- URL: `~/keyman/web/testing/spacebar-text/index.html`