Default UI redesign (#7429)

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* Update default.sidebar.vue

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* Update sticky-sidebar.ts

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* Update messaging-room.form.vue

* Update timeline.vue

src/client/scripts/theme.ts

@@ -17,6 +17,7 @@ export const themeProps = Object.keys(lightTheme.props).filter(key => !key.start
 export const builtinThemes = [
 	require('../themes/l-light.json5'),
 	require('../themes/l-apricot.json5'),
+	require('../themes/l-rainy.json5'),
 
 	require('../themes/d-dark.json5'),
 	require('../themes/d-persimmon.json5'),