docs: make get started instructions around themes more clear

[justsml]

I think this looks good. Just did a casual check, was hoping I could get more eyes on it.

Thanks in advance ❤️

[welcome]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-io]

Codecov Report

Merging #3713 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3713      +/-   ##
==========================================
- Coverage   99.84%   99.83%   -0.01%     
==========================================
  Files         178      175       -3     
  Lines        3150     3120      -30     
==========================================
- Hits         3145     3115      -30     
  Misses          5        5

Impacted Files	Coverage Δ
src/modules/Popup/Popup.js	98.68% <0%> (-0.04%)	⬇️
src/modules/Transition/TransitionGroup.js	100% <0%> (ø)	⬆️
src/collections/Form/FormTextArea.js	100% <0%> (ø)	⬆️
src/collections/Form/FormRadio.js	100% <0%> (ø)	⬆️
src/modules/Checkbox/Checkbox.js	100% <0%> (ø)	⬆️
src/modules/Sidebar/Sidebar.js	100% <0%> (ø)	⬆️
src/collections/Form/FormInput.js	100% <0%> (ø)	⬆️
src/collections/Form/FormDropdown.js	100% <0%> (ø)	⬆️
src/collections/Form/FormField.js	100% <0%> (ø)	⬆️
src/modules/Dropdown/DropdownItem.js	100% <0%> (ø)	⬆️
... and 28 more

---

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[levithomason]

I really like the split here in explanations 👍 Keep it coming.

[justsml]

Hi @levithomason let me know if you want to see anything else added/changed.

I'm happy to jump on it.

[codecov-commenter]

Codecov Report

Merging #3713 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3713   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         184      184           
  Lines        3218     3218           
=======================================
  Hits         3213     3213           
  Misses          5        5           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[layershifter]

I simplified markup a bit, LGTM 👍

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉