src/vs/workbench/browser/parts/activitybar/activitybarPart.ts/ActivitybarPart/updateStyles
class ActivitybarPart:	override updateStyles(): void {
		super.updateStyles();

		const container = assertIsDefined(this.getContainer());
		const background = this.getColor(ACTIVITY_BAR_BACKGROUND) || '';
		container.style.backgroundColor = background;

		const borderColor = this.getColor(ACTIVITY_BAR_BORDER) || this.getColor(contrastBorder) || '';
		container.classList.toggle('bordered', !!borderColor);
		container.style.borderColor = borderColor ? borderColor : '';
	}