You can use the {StaticResource} markup extension
<RelativePanel x:Name="MymusicSubPanel" Background="{ThemeResource SystemControlTranslucentHighBrush}" ... >
...
</RelativePanel>
And you can use different color to your Theme.
<SolidColorBrush x:Key="SystemBackgroundAltHighBrush" Color="#FFF0F0F0"/>
How to use the Theme?You can use it in the Page resources.
<Page.Resources>
<ResourceDictionary>
<ResourceDictionary.ThemeDictionaries>
<ResourceDictionary Source="LightThemeDictionary.xaml" x:Key="Light"/>
<ResourceDictionary Source="DarkThemeDictionary.xaml" x:Key="Dark"/>
<ResourceDictionary Source="HighContrastBlackDictionary.xaml" x:Key="HighContrastBlack"/>
...
</ResourceDictionary.ThemeDictionaries>
</ResourceDictionary>
</Page.Resources>
You can use the Theme,you write the back is black and text foreground color is white.
https://embracez.xyz/xaml-uwp-themes/