Draft: Update theme

[scharlottej13]

Re dask/community#220

This is a start to updating the theme, most changes so far are for fixing the landing page and then I'll move on to the posts. Opening up a draft PR since I'll be out next week.

Notes from the design team:

The pdf mockup: https://www.dropbox.com/s/bnmu6isfthrb0ur/Dask-Blog-Design.pdf?dl=0
Page one is the main blog page, page 2 is the individual blog page, and page 3 is the tags. I’ve tried to keep the structure as close to the existing page, while adding in a few of our features from the Webflow site design.

Page 1: Main Blog page

• The red bar is the hover state, with white text
• Since we’re using the site wide nav, I added the link to the tag view under the blog title.

Page 2: Individual Blog Page

• You can use the same hierarchy for headings, body text and code blocks that are used on the documentation page.

Page 3: Tags Page

• Same style treatment as the main blog page.
• Could we remove the jump links? They currently take up a lot of vertical height on the page.

Nav Bar: https://www.dask.org/navigation
Footer: https://www.dask.org/footer

Type treatments & colors: https://www.dropbox.com/s/ddlpz82f3h6ju0c/Dask-Doc%20Portal%20Spec%20Sheet.pdf?dl=0

Background Grid SVG: https://www.dropbox.com/s/qdvyf63ny8cea76/grid-repeating%20pattern.svg?dl=0

This is all in the doc portal folder: https://www.dropbox.com/sh/3z5vvar4sk0tgz4/AAA36Q8foEDJ-lHX5emmoaLya?dl=0

cc @jacobtomlinson