docs(theming): add a note/section on CSS variable limitations #167

URL:
https://beta.ionicframework.com/docs/theming/advanced

We already have a section on the alpha problem, but maybe we could expand on this (or add in CSS variables section) to include some known limitations of CSS. We should include this issue with Safari and rgb: https://bugs.webkit.org/show_bug.cgi?id=185940