Add support for color spaces other than truecolor

[lunrenyi]

你好，embark-theme 的作者

在不支持 true color 的终端中，embark-theme 的使用效果很差，比如 mac 自带的终端

最差的情况下会导致无法看清楚编辑内容：

附录：
在支持 true color 的终端中的效果

[skbolton]

It has been a todo for a long time. I currently don't have a need for anything other than truecolor support so it hasn't been done yet. PR's are always welcome.

[lunrenyi]

Sure, thank you for your reply.

[skbolton]

I'll keep this issue around to track things. If it feels like something a handful of people want I can do a first pass at making it happen.