chore: Country selector button invisible in dark mode

[g123k]

Hi everyone!

Here's a very minor issue.
In the country selector and in dark mode, the "Save" button was invisible:

Before/After:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 6.50%. Comparing base (4d9c7fc) to head (2a1020f).
Report is 546 commits behind head on develop.

Files with missing lines	Patch %	Lines
...preferences/country_selector/country_selector.dart	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #5979      +/-   ##
==========================================
- Coverage     9.54%   6.50%   -3.05%     
==========================================
  Files          325     441     +116     
  Lines        16411   24974    +8563     
==========================================
+ Hits          1567    1624      +57     
- Misses       14844   23350    +8506     

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