fix: apply details background only when details is an overlay

[sissbruecker]

Description

For 24.8 and 24.9, #9669 changed the MDL base styles so that a background color is applied to the details even if the details are not shown as an overlay. That seems like a mistake as the v25 base styles only apply a background when the details are shown as an overlay:

web-components/packages/master-detail-layout/src/styles/vaadin-master-detail-layout-base-styles.js

Lines 146 to 149 in 5a13239

	:host(:is([drawer], [stack])) [part='detail'] {
	background: var(--vaadin-master-detail-layout-detail-background, var(--vaadin-background-color));
	box-shadow: var(--vaadin-master-detail-layout-detail-shadow, 0 0 20px 0 rgba(0, 0, 0, 0.3));
	}

This moves the background color to the more specific rule that only applies when rendering details as an overlay.

Fixes #10299

Type of change

• Bugfix

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jouni]

I think the reason for moving the background color was that even in split mode, the master and detail parts can cross-fade, and the detail part shows on top of the initial master view briefly. But perhaps that’s a concern themes should worry about, not the base styles.

[sissbruecker]

We can revert this and update themes to use the base background color on non-overlay details as well if this turns out to be an issue.