add alternative theme usage syntax

[hipstersmoothie]

What Changed

Add new theme("VARIABLE_NAME") syntax.

Why

It is easier to read.

Todo:

• Add tests
• Add docs

Published PR with canary version: 2.7.0-canary.53.684

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@322893c). Click here to learn what that means.
The diff coverage is 85.00%.

@@            Coverage Diff            @@
##             master      #53   +/-   ##
=========================================
  Coverage          ?   88.11%           
=========================================
  Files             ?        5           
  Lines             ?      345           
  Branches          ?       89           
=========================================
  Hits              ?      304           
  Misses            ?       25           
  Partials          ?       16           

Impacted Files	Coverage Δ
src/index.ts	83.78% <50.00%> (ø)
src/common/index.ts	79.77% <70.00%> (ø)
src/legacy/index.ts	96.38% <88.23%> (ø)
src/modern/index.ts	89.39% <92.68%> (ø)

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[hipstersmoothie]

🚀 PR was released in v2.7.0 🚀