Added styled-components theme provider

[arronhunt]

Resolves #80

The goal is to convert the current method of references theme variables to use styled-components' theme provider. This will allow for the default theme to be overridden and allow for more customization for admin-bro.

TODO:

• Use <ThemeProvider> and theme props for variables
• Allow for a theme to be passed down via AdminBroOptions

[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 2.63%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   58.37%   55.74%   -2.64%     
==========================================
  Files         113      114       +1     
  Lines        2047     2149     +102     
  Branches      191      188       -3     
==========================================
+ Hits         1195     1198       +3     
- Misses        821      921     +100     
+ Partials       31       30       -1

Impacted Files	Coverage Δ
src/frontend/components/ui/loader.jsx	71.42% <0%> (-14.29%)	⬇️
src/frontend/components/ui/label.jsx	72.72% <0%> (-27.28%)	⬇️
src/frontend/components/login/index.jsx	41.66% <0%> (-28.34%)	⬇️
src/frontend/components/ui/property-in-filter.jsx	66.66% <0%> (-16.67%)	⬇️
src/frontend/components/app/record-in-list.jsx	64.7% <0%> (-11.77%)	⬇️
src/frontend/components/ui/paginate.jsx	54.54% <0%> (-6.75%)	⬇️
src/frontend/components/ui/styled-button.jsx	42.85% <0%> (-47.15%)	⬇️
src/frontend/components/ui/dropdown.jsx	60% <0%> (-13.34%)	⬇️
src/frontend/components/app/breadcrumbs.jsx	40% <0%> (-12.39%)	⬇️
src/frontend/components/ui/table.jsx	55.55% <0%> (-44.45%)	⬇️
... and 40 more

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[arronhunt]

@wojtek-krysiak I also wanted to propose a few changes to the overall theme/design that I personally feel would be beneficial.

1. Changing default variable sizes to use the 8pt grid system
2. Convert to rem units instead of pixel units
3. Changing some of the color values to improve contrast to meet accessibility standards (see image below)

These are all changes that can easily be made to the src/styles/variables.js file and would greatly improve responsiveness and a11y of the application.

[wojtek-krysiak]

@arronhunt next Friday at SoftwareBrothers we are hosting a hackaton and designers told me that they would like to update the design of AdminBro (on hackaton) :) - I already passed your comments to them so they will make an update.

[wojtek-krysiak]

@arronhunt I've merged that to v0.9 branch. I will release it along with other bigger changes.