feat: Enable automatic Christmas theme in December

[faucomte97]

This change is 

[SKairinos]

Reviewed 26 of 27 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @faucomte97)

---

game/decor.py line 70 at r2 (raw file):

        # FUTURE: add external branding option
        url="decor/grass/cfc.svg",
        xmas_url="decor/grass/cfc.svg",

not snow/cfc.svg?

[faucomte97]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @SKairinos)

---

game/decor.py line 70 at r2 (raw file):

Previously, SKairinos (Stefan Kairinos) wrote…

not snow/cfc.svg?

Good catch, thanks

[codecov]

Codecov Report

Merging #1538 (1cf947e) into master (31dde05) will increase coverage by 0.01%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head 1cf947e differs from pull request most recent head 0776a97. Consider uploading reports for the commit 0776a97 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1538      +/-   ##
==========================================
+ Coverage   91.79%   91.80%   +0.01%     
==========================================
  Files         110      110              
  Lines        5860     5871      +11     
==========================================
+ Hits         5379     5390      +11     
  Misses        481      481              

Files	Coverage Δ
game/character.py	100.00% <ø> (ø)
game/decor.py	78.78% <100.00%> (+0.66%)	⬆️
game/level_management.py	89.77% <ø> (ø)
game/theme.py	100.00% <ø> (ø)
game/views/level.py	74.44% <100.00%> (+1.17%)	⬆️

[SKairinos]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @faucomte97)