perf(conf.d): skip theme colors in non-interactive shells

fish_color_* is consumed only by the syntax highlighter. The guard sits
below the existing cleanup branch so stale-FZF_DEFAULT_OPTS cleanup keeps
running where it does today; the FZF value itself is a persisted universal
and survives regardless.

conf.d/theme.fish

@@ -20,6 +20,11 @@ if not __fish_config_op_enabled (status basename)
     return
 end
 
+# Below the cleanup block on purpose: that branch erases a stale universal
+# FZF_DEFAULT_OPTS and must keep running wherever it runs today. Everything
+# past here is fish_color_* for the syntax highlighter, interactive-only.
+status is-interactive; or return
+
 #   ────────────────────── Syntax highlighting colors ──────────────────────
 set --global fish_color_autosuggestion 6c7086
 set --global fish_color_cancel f38ba8