core: setup theme extensions

[wizlif]

Refactor of #362
Closes #344

[codecov]

Codecov Report

Merging #373 (7b69c92) into development (6528ad6) will increase coverage by 0.40%.
The diff coverage is 69.41%.

@@               Coverage Diff               @@
##           development     #373      +/-   ##
===============================================
+ Coverage        65.63%   66.04%   +0.40%     
===============================================
  Files              161      167       +6     
  Lines             4065     4532     +467     
===============================================
+ Hits              2668     2993     +325     
- Misses            1397     1539     +142     

Impacted Files	Coverage Δ
lib/presentation/auth/auth_screen.dart	1.81% <0.00%> (-0.19%)	⬇️
lib/presentation/auth/unauthenticated_screen.dart	0.00% <0.00%> (ø)
lib/presentation/core/app_widget.dart	0.00% <0.00%> (ø)
.../crowdaction_browse/crowdaction_browse_screen.dart	0.00% <0.00%> (ø)
...rowdaction_comments/crowdaction_comments_page.dart	2.50% <0.00%> (-0.07%)	⬇️
...action_comments/parts/comment_appbar_delegate.dart	3.70% <0.00%> (-0.15%)	⬇️
...ction/crowdaction_comments/parts/comment_item.dart	0.00% <0.00%> (ø)
.../crowdaction_comments/parts/comment_textfield.dart	10.00% <0.00%> (-10.00%)	⬇️
...ion/crowdaction_comments/widgets/flag_comment.dart	0.00% <0.00%> (ø)
...rowdaction_details/crowdaction_details_screen.dart	0.00% <0.00%> (ø)
... and 74 more

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[wizlif]

Themes will be handled from collaction_common package