plugin: wip styling

src/mol-plugin/context.ts

src/mol-plugin/skin/base/components/controls.scss

src/mol-plugin/skin/base/layout/common.scss

src/mol-plugin/skin/base/variables.scss

src/mol-plugin/state/transforms/visuals.ts

src/mol-plugin/ui/controls/common.tsx

src/mol-plugin/ui/controls/parameters.tsx

src/mol-plugin/ui/controls/slider.tsx

src/mol-plugin/ui/plugin.tsx

src/mol-plugin/ui/viewport.tsx