move dark-mode styling into dashboard.css

[stklcode]

Dark mode styling has been added inline using the "doing_dark_mode" hook which is no longer called in current plugin versions.

Include the CSS definition directly into dashboard.css and deprecate the hook for adding inline styles.

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resolves #264

However: This does not apply to all "dark-mode" plugins and not even to all themes, because the icon has two different shades of gray and is not affected by most stylings that assume Dashicons are used here. We might prefer a different approach like #277

[sonarcloud[bot]]

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication