Remove link to compromised styled-system.com domain (#1364)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ryan-roemer <445557+ryan-roemer@users.noreply.github.com>

Author: Copilot

docs/themes.md

@@ -6,7 +6,7 @@ sidebar_position: 8
 
 # Theme System
 
-Spectacle has a robust theme system that is built upon [styled system](https://styled-system.com/theme-specification).
+Spectacle has a robust theme system that is built upon styled system.
 
 A theme is a 2-level deep object of labeled theme keys and CSS property object values, which is passed directly to the `Deck` component.