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