[DEV-656] color config proposal

[ObservedObserver]

1. since there is already a themeConfig, the name of color config sounds confusing. My suggestion is change the param to "UITheme"
2. support deep merge of variables. If there is no ui theme config design tool, it will be more dev-friendly to allow dev to overrides some css variables instead of the whole setting

_{From SyncLinear.com | DEV-656}