Specify a single box-shadow using Two, three, or four <length> values.
- If only two values are given, they are interpreted as <offset-x> and <offset-y> values.
- If a third value is given, it is interpreted as a <blur-radius>.
- If a fourth value is given, it is interpreted as a <spread-radius>.
- Optionally, the inset keyword.
- Optionally, a <color> value.
To specify multiple shadows, provide a comma-separated list of shadows.