... system, in most cases it is black, but on Windows 10 and macOS with dark theme it is white. This tray icon can be used to call clock configuration dialog. Also this dialog can be called using clock context menu.
You can create your own skin! See this link for details: https://sourceforge.net/p/digitalclock4/wiki/How%20to%20create%20skin/
NOTE: do NOT download any .rar or .exe files marked as my software, these files are made NOT by me and I'm not responsible for their content!