Fixes #1853

Thanks @towerofnix!

- Auditing color CSS and componet CSS to make consistent.
- Made a first pass of changes to increase contrast to elements that previously were problematic

* Close button component: large: added similar transition to add button
* Filter component: added autofocs, made full width

Affects SpriteSelector Pane, StageSelector Pane, Stage, MenuBar, Libraries Modal, Add/Import buttons, for Chrome, Ffx + Safari on Sierra.

Fixes #162

Refactored towards better convention for linting.

* Design pass on libraries to make sprites look more like they do in the SpriteSelector. Fixed scrolling overflow when reaching the bottom
* Started a colors variables file
* Added a light transform on sprite item hover

Related #12

* Covers parts of these issues: GUI layout panes (#69), sprite libraries (#12), import buttons (#64), menu bar (#67), and sprite info area (#53)

~ Refactored layout pane structure by removing `display: flex` from `.box`, by default. Declaring flex instead only where specifically needed. This helped fix layout quirks, and should help us avoid hard to find issues related to nested flex boxes

~ Added descriptive classnames to boxes/containers/wrappers. Moved more padding, margins + layout into these wrappers to help with component reusability.

~ Set up a spacer unit variable inside `gui.css`, to start keeping consistent padding, margins and rounded corners between panes. Used [CSS/PostCSS Modules](https://github.com/css-modules/postcss-modules-values) to avoid SCSS dependency.

~ Refactored inline styles from JSX, into CSS files. 2 reasons: a) prepping for reuse of CSS Module variables. b) Feels easier to debu...

The inline css method gives us nothing that CSS modules can't give us. This puts the project more in line with the way www works, and makes CSS actually readable.

Using class names gives us full control of the modal styles.