fix: add missing custom.css file for docs build

Resolves Vite build error where custom.css was referenced but missing.

Author: avrabe

docs-site/src/styles/custom.css

@@ -0,0 +1,6 @@
+/* Custom styles for Bazel File Operations Component documentation */
+
+:root {
+  --sl-color-accent: #2563eb;
+  --sl-color-accent-high: #1e40af;
+}