✨ chore: provide defaults for theming styles

[Mastersam07]

• defaults for theme style
• light and dark constructor on theme style

Closes #10

[codecov]

Codecov Report

Merging #11 (9d8d190) into dev (86f4151) will decrease coverage by 2.38%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##              dev      #11      +/-   ##
==========================================
- Coverage   92.20%   89.82%   -2.38%     
==========================================
  Files           7        7              
  Lines         218      226       +8     
==========================================
+ Hits          201      203       +2     
- Misses         17       23       +6     

Impacted Files	Coverage Δ
lib/src/themes/theme_data.dart	61.11% <20.00%> (-17.47%)	⬇️
lib/src/themes/button_style.dart	89.28% <50.00%> (-3.03%)	⬇️
lib/src/themes/colors.dart	85.00% <50.00%> (-3.89%)	⬇️

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