fix(www): fix color-scheme and combine duplicate html declarations

[jdegand]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Update styles in new docs. Also, there was a duplicate html block so I combined those styles into the initial html block.

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Color-scheme property is not properly set.

Closes #4854

What is the new behavior?

Default color-scheme is set.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

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[markostanimirovic]

LGTM 👍 Thanks @jdegand!