src/vs/workbench/services/themes/common/themeExtensionPoints.ts/ThemeRegistry/findThemeById
class ThemeRegistry:	public findThemeById(themeId: string, defaultId?: string): T | undefined {
		if (this.builtInTheme && this.builtInTheme.id === themeId) {
			return this.builtInTheme;
		}
		const allThemes = this.getThemes();
		let defaultTheme: T | undefined = undefined;
		for (const t of allThemes) {
			if (t.id === themeId) {
				return t;
			}
			if (t.id === defaultId) {
				defaultTheme = t;
			}
		}
		return defaultTheme;
	}