Support group header hover and regular bg

[rferreira98]

@jassmith

With this PR we'll be able to customize background colors for group headers and also change the hover color as well without affecting regular headers.

[hakobyanheghine]

Hey @jassmith ,

When can we get this merged? This would be very helpful for my case as well!

Thanks!

[sergeynersesyan]

This is very helpful feature. Waiting for it to be merged

[Copilot]

Pull Request Overview

This PR adds support for custom background and hover colors specifically for group headers without affecting regular headers.

• Introduce two new theme properties: bgGroupHeader and bgGroupHeaderHovered
• Update the drawing logic to use these new properties with proper fallbacks
• Expose corresponding CSS variables and document the bgGroupHeader variable in the theming guide

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/internal/data-grid/render/data-grid-render.header.ts	Use bgGroupHeader/bgGroupHeaderHovered in fillColor with fallback to bgHeader
packages/core/src/docs/08-theming.stories.tsx	Added a docs entry for the bgGroupHeader CSS variable
packages/core/src/common/styles.ts	Added CSS variables and theme interface fields for group header backgrounds

Comments suppressed due to low confidence (2)

packages/core/src/docs/08-theming.stories.tsx:257

• The theming docs lack an entry for the new bgGroupHeaderHovered CSS variable (--gdg-bg-group-header-hovered). Please add a row describing its purpose and default fallback behavior.

| bgGroupHeader | --gdg-bg-group-header | string \\| undefined | The group header background color, if none provided the `bgHeader` is used instead. |

packages/core/src/internal/data-grid/render/data-grid-render.header.ts:185

• New behavior for group header backgrounds and hover styling is introduced here. Please add or update unit/integration tests to verify that bgGroupHeader and bgGroupHeaderHovered are applied correctly under various override scenarios.

export function drawGroups(

[lukasmasuch]

LGTM 👍