update docs for background color in "color" theme

packages/storybook/src/nimble/banner/banner.stories.ts

@@ -1,6 +1,7 @@
 import { html, when } from '@microsoft/fast-element';
 import { withActions } from '@storybook/addon-actions/decorator';
 import type { HtmlRenderer, Meta, StoryObj } from '@storybook/html';
+import { Theme } from '../../../../nimble-components/src/theme-provider/types';
 import { iconKeyTag } from '../../../../nimble-components/src/icons/key';
 import { buttonTag } from '../../../../nimble-components/src/button';
 import { ButtonAppearance, ButtonAppearanceVariant } from '../../../../nimble-components/src/button/types';
@@ -92,7 +93,7 @@ export const _banner: StoryObj<BannerArgs> = {
             options: Object.keys(BannerSeverity),
             control: { type: 'radio' },
             description:
-                'Severity of the message presented by the banner. Controls the displayed color and icon. If not set, the banner has a neutral appearance.'
+                `Severity of the message presented by the banner. Controls the icon displayed within the banner and, in themes other than \`${Theme.color}\`, controls the background color of the banner. If not set, the banner has a neutral appearance.`
         },
         action: {
             options: Object.values(ActionType),