smplayer | Free Media Player for Windows , Linux and Mac OS | Media Player library

AVI is not meant to be used with HEVC.
Use a modern output container format such as MP4, MOV, or MKV.

A workaround is to add the -vtag hvc1 output option.

For playback speed, the only option available in the LibVLC API is this:

There is no direct method can bring the tab into view, if you want to achieve it, you can find the ScrollViewer which contains those tabs and then use ChangeView method to scroll it.

Based on the style of TabView, there is a ScrollViewer in TabView, you can try to find it in code-behind and then use ChangeView method to scroll it. In addition, when you try to get the scrollViewer, you need to implement it in loaded event, since in the OnNavigatedTo event, the control has not been loaded completely. For example:

Advisably you should he handling this using ThemeResource's in XAML, not in code, see: https://docs.microsoft.com/en-us/windows/uwp/xaml-platform/themeresource-markup-extension

But to answer your question this is expected behaviour. ElementTheme.Default simply means the element has not had it's theme overridden and is using the default app theme. The other two values mean the element has specifically had it's theme overridden. App.Current.RequestedTheme gives the application's actual theme. FrameworkElement.RequestedTheme will always have a value of Default unless you explicitly set it to something else on that specific element. All of it's children will still have the value Default.

Note, you should compare against ActualTheme, not RequestedTheme, as a parent may have caused it to be using a different theme than the application if it's value is still ElementTheme.Default.

A method like below might help you to get a proper value of Light/Dark.