monaco-editor/website/playground/new-samples/customizing-the-appearence/scrollbars/sample.css

/* Make horizontal scrollbar, decorations overview ruler and vertical scrollbar arrows opaque */
.monaco-editor .monaco-scrollable-element .scrollbar.horizontal,
.monaco-editor .decorationsOverviewRuler,
.monaco-editor .monaco-scrollable-element .scrollbar.vertical .arrow-background {
	background: rgba(230, 230, 230, 255);
}
/* Make vertical scrollbar transparent to allow decorations overview ruler to be visible */
.monaco-editor .monaco-scrollable-element .scrollbar.vertical {
	background: rgba(0, 0, 0, 0);
}