fix nb-sticky css (microsoft#189350)

Author: Yoyokrazy

src/vs/workbench/contrib/notebook/browser/media/notebookEditorStickyScroll.css

@@ -11,6 +11,13 @@
 	width: 100%;
 	font-family: var(--notebook-cell-input-preview-font-family);
 }
+
+.monaco-workbench.hc-light .notebookOverlay .notebook-sticky-scroll-container,
+.monaco-workbench.hc-black .notebookOverlay .notebook-sticky-scroll-container {
+	background-color: var(--vscode-editorStickyScroll-background);
+	border-bottom: 1px solid var(--vscode-contrastBorder);
+}
+
 .monaco-workbench
 	.notebookOverlay
 	.notebook-sticky-scroll-container