Use variables for all colors

[severo]

No breaking change.

Use variables for all colors, to make it easier to change the theme colors. I will help in particular, to implement a dark theme (asked here hyparam/hyparquet#114 and here: source-cooperative/parquet-table#4).

We might want to reduce the number of colors and harmonize as a follow-up.

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Also, fixes the color of the column menu items (removes the need to fix it, as in https://github.com/hyparam/demos/blob/2a45a13b3294613bf9f97718434baf87624becdb/hyparquet/src/index.css#L320-L322, when it's overridden by the default button color)

[Copilot]

Pull Request Overview

This PR refactors hardcoded color values throughout the CSS to use CSS custom properties (variables), improving maintainability and theming capabilities. The change also moves the CSS module import path and removes unused animation keyframes.

• Introduces comprehensive CSS color variables organized by text, background, and border colors
• Replaces all hardcoded color values with semantic variable references
• Consolidates color definitions to enable easier theme customization

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/components/HighTable/HighTable.tsx	Updates CSS module import path to reference moved stylesheet
src/HighTable.module.css	Defines color variables and replaces hardcoded colors throughout, removes unused shimmer animation

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[severo]

fyi @platypii. I didn't ask for a review because it does not break anything.

Note that it makes very clear that we use (too?) many colors. What about reducing it in the default theme.