Update README with example of how to create a custom theme

[Suplanus]

I found no way to declare a custom theme base on colored theme. The class ConsoleHtmlColor is set to internal so I can't use it. Any idea, or should I send a PR?

[augustoproiete]

You don't really need ConsoleHtmlColor in order to create a custom theme... Colors are just HTML color code strings. E.g.:

var customTheme = new RichTextBoxConsoleTheme
(
    new Dictionary<RichTextBoxThemeStyle, RichTextBoxConsoleThemeStyle>
    {
        [RichTextBoxThemeStyle.Text] = new RichTextBoxConsoleThemeStyle { Foreground = "#ffffff" },
        [RichTextBoxThemeStyle.SecondaryText] = new RichTextBoxConsoleThemeStyle { Foreground = "#c0c0c0" },
        // etc.
    }
);

That said, if the ConsoleHtmlColor class is useful for creating custom themes, I'd take a PR to make the class public.

[Suplanus]

Ah, great... thanks 💖 So I think its not needed to make it public. I will make an example for white background and take it in the readme as PR.