fix(#31): replace the transparent value with a solid color token

[svmukhin]

Closes #31

Replace the transparent value with a solid color token:

@media (prefers-color-scheme: dark) {
  input:not([type="checkbox"]):not([type="radio"]),
  textarea,
  select {
    background: var(--bg-secondary);
    border-color: var(--border);
  }
}

Using an opaque --bg-secondary (#161b22) ensures the native dropdown inherits a dark background regardless of how the OS composites it.

Additional Notes

• color-scheme: dark on select was also tested but did not reliably fix the issue across browsers.
• Setting background-color on option elements has no effect — native dropdowns ignore it on most platforms.
• The solid token approach is the most cross-browser compatible solution.