Issues with themes

[GriffinRichards]

• Poor tab styling in dark modes (See Improve tab style in dark modes #561)
• Placeholder text color isn't updated when the theme changes (See Placeholder text color doesn't update correctly with theme change #560). QPalette::PlaceholderText or placeholder-text-color are useful for this
• URL color contrast on dark themes is still bad on Windows. Fixed: eecdd8c
• We have no actual "Light" theme mode, just a "Default" which can be overridden by the system theme. A consequence for this is that it's not possible for a user to have Porymap in a Light theme if it disagrees with their system theme. If we accept system themes we may wish to detect dark mode and switch to our dark mode stylesheet automatically instead.
• On newer versions of macOS the Default theme doesn't change the appearance of QComboBox dropdown handles if they're disabled, so they appear identical to enabled QComboBoxs with placeholder text:

• Changing to the Default theme retains some incorrect colors. See image (apparently just QScrollArea on Windows, not reproducible on macOS)