[feature request] Optional monochrome / grayscale menubar icon

First, thank you for MacThrottle—it is a very useful utility.

I would like to request an option to make the menubar thermometer icon monochrome or grayscale, instead of the current colorful version.

On modern macOS setups, most menubar icons are monochrome to maintain visual consistency. The current colorful thermometer stands out noticeably and breaks that consistency, especially for users who keep a minimal or system-styled menubar.

Suggested implementation options (any would be fine):
- A preference toggle: Colorful icon vs Monochrome icon
- Automatically follow macOS menubar appearance (template image / system tint)
- Provide a grayscale alternative icon asset

This is purely a UI/UX request; the existing functionality is already solid.