fix(theme): improves legibility for overlays

[dzucconi]

• Improves the contrast for text using gradient overlays
  • Gradient easing via https://larsenwork.com/easing-gradients/

• Uses the themed backdrop for drawers

  📦 Published PR as canary version: Canary Versions

  
  :sparkles: Test out this PR locally via: ```bash npm install @artsy/palette-charts@37.12.2-canary.1389.30758.0 npm install @artsy/palette-tokens@6.0.3-canary.1389.30758.0 npm install @artsy/palette@38.12.2-canary.1389.30758.0 # or yarn add @artsy/palette-charts@37.12.2-canary.1389.30758.0 yarn add @artsy/palette-tokens@6.0.3-canary.1389.30758.0 yarn add @artsy/palette@38.12.2-canary.1389.30758.0 ```

[artsyit]

:rocket: PR was released in @artsy/palette-charts@37.12.2, @artsy/palette-tokens@6.0.3, @artsy/palette@38.12.2 :rocket: