fix: add base styles for month/year select dropdowns

[martijnrusschen]

Summary

Added styling for the month/year <select> dropdown elements that were previously unstyled:

• .react-datepicker__month-select
• .react-datepicker__year-select
• .react-datepicker__month-year-select

Problem

When using showMonthDropdown, showYearDropdown, or showMonthYearDropdown with dropdownMode="select", the native <select> elements had no styling and used browser defaults. This caused issues:

• Dark text on dark background in dark mode themes
• Inconsistent appearance with the rest of the datepicker
• No visual integration with custom themes

Solution

Added base styles that:

• Use transparent background to inherit from parent container
• Use color: inherit to match parent's text color
• Apply consistent border, border-radius, and font styling from the datepicker's variables
• Include focus-visible outline for accessibility
• Use cursor: pointer for better UX

This approach allows the dropdowns to work correctly with both light and dark themes by inheriting colors from their parent elements, rather than hardcoding specific color values.

Generated CSS

.react-datepicker__month-select,
.react-datepicker__year-select,
.react-datepicker__month-year-select {
  background-color: transparent;
  border: 1px solid #aeaeae;
  border-radius: 0.3rem;
  color: inherit;
  cursor: pointer;
  font-family: inherit;
  font-size: inherit;
  padding: 2px;
}
.react-datepicker__month-select:focus-visible,
.react-datepicker__year-select:focus-visible,
.react-datepicker__month-year-select:focus-visible {
  outline: auto 1px;
}

Test Plan

• CSS builds without errors
• All 1466 tests pass
• Linting passes

Fixes #5786

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.18%. Comparing base (9e4795d) to head (9f736c2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6169      +/-   ##
==========================================
- Coverage   99.26%   99.18%   -0.08%     
==========================================
  Files          30       30              
  Lines        3801     3801              
  Branches     1652     1652              
==========================================
- Hits         3773     3770       -3     
- Misses         27       29       +2     
- Partials        1        2       +1     

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