Refactor sash hover border to use CSS var (microsoft#165516)

Ref microsoft#165169

Author: rzhao271

src/vs/base/browser/ui/sash/sash.css

@@ -111,6 +111,11 @@
 	background: transparent;
 }
 
+.monaco-sash.hover:before,
+.monaco-sash.active:before {
+	background: var(--vscode-sash-hoverBorder);
+}
+
 .monaco-sash.vertical:before {
 	width: var(--sash-hover-size);
 	left: calc(50% - (var(--sash-hover-size) / 2));

src/vs/workbench/contrib/sash/browser/sash.contribution.ts

@@ -10,8 +10,6 @@ import { Registry } from 'vs/platform/registry/common/platform';
 import { workbenchConfigurationNodeBase } from 'vs/workbench/common/configuration';
 import { IWorkbenchContributionsRegistry, Extensions as WorkbenchExtensions } from 'vs/workbench/common/contributions';
 import { SashSettingsController } from 'vs/workbench/contrib/sash/browser/sash';
-import { registerThemingParticipant } from 'vs/platform/theme/common/themeService';
-import { sashHoverBorder } from 'vs/platform/theme/common/colorRegistry';
 import { isIOS } from 'vs/base/common/platform';
 
 // Sash size contribution
@@ -39,13 +37,3 @@ Registry.as<IConfigurationRegistry>(ConfigurationExtensions.Configuration)
 			},
 		}
 	});
-
-registerThemingParticipant((theme, collector) => {
-	const sashHoverBorderColor = theme.getColor(sashHoverBorder);
-	collector.addRule(`
-		.monaco-sash.hover:before,
-		.monaco-sash.active:before {
-			background: ${sashHoverBorderColor};
-		}
-	`);
-});