chore: update design

[baseballyama]

related: #253

I updated below points.

• Fix background color -> I stop to use gradation.
• change header behavior like SvelteKit web site -> Now header has fixed position.
• Left side scroll bar's color. (Maybe SvelteKit web site7s color is better?) -> I followed Svelte docs color.

I'm not designer, so I don't have ability to create good design, but I think this PR is bit improved.
I'm open for your opinion.

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[baseballyama]

--secondary-color is not used at all. So I updated this color according to Svelte site7s color.

[ota-meshi]

Great Job! Thank you!