Windows
Hold Alt and type 233 on the numeric keypad.
Greek
Copy the character, or use the shortcut that matches your keyboard and app.
Hold Alt and type 233 on the numeric keypad.
Character Viewer → theta
Type 03B8 then Alt + X
Insert → Special characters → theta
Click the keycap, then paste with Ctrl+V or Cmd+V.
Num Lock on, hold Alt, type 233, release Alt.
Use the Unicode, HTML, CSS, or JS value below in documents and stylesheets.
U+03B8θθ\03B8\u{03B8}Alt + 233