Theme as a modifier results in the need to increase specificity

Continuing the conversation with @veged, I want to discuss the issue with theme as a modifier.

Here are cases:
1. To make some block invisible with modifier or mix, user should often use `mustDeps` on lib's theme or somehow increase specificity as there are `display` properties in _theme_islands (e.g. https://github.com/bem/bem-components/blob/v2/design/common.blocks/button/_theme/button_theme_islands.styl#L7).
2. The same issue appear when user wants to create layout with the help of mixes with some layout block as such properties as `margin` and `padding` are set as well (https://github.com/bem/bem-components/blob/v2/design/common.blocks/button/_theme/button_theme_islands.styl#L9-L10).

Maybe someone will find the way to make it better?

Start by reading design/common.blocks/button/_theme/button_theme_islands.styl, especially the referenced display, margin, and padding rules. Review the two examples in the issue and the linked conversation to determine the intended behavior; done means theme modifiers and layout mixes no longer require extra specificity or mustDeps workarounds.

Written by the indexing model from the issue text.