Migrate to fluent theme

[maraf]

https://learn.microsoft.com/en-us/dotnet/desktop/wpf/whats-new/net90?view=netdesktop-9.0

Let's try to match them of PowerToys Run utility, but possibly with smaller scale

[maraf]

Work in progress in https://github.com/neptuo/Productivity.SnippetManager/tree/fluentTheme.
Postponing for now as the result isn't appealing at the moment.