Update themes.md

[BritV8]

using css, powershell cannot handle the hyphen in the name (font-style) added ' ' to your CSS examples e.g. 'font-size' = '20px' 'font-style' = 'italic' Would error in powershell 5.1 on UD 2.9.0 My specific error was using

'.btn' = @{
    color = '#FFFFFF'
    'background-color' = '#004D57'
}
'.btn:hover' = @{
    color = '#FFFFFF'
    'background-color' = '#002f36'
}