fix: improve back-to-top button contrast (Conflict Resolution for #325)

src/components/BackToTop.tsx

@@ -38,7 +38,7 @@ const BackToTop = (): JSX.Element | null => {
           animate={{ opacity: 1, y: 0 }}
           exit={{ opacity: 0, y: 50 }}
           transition={{ duration: 0.3 }}
-          className="fixed bottom-28 right-11 z-50 bg-black text-white px-4 py-3 rounded-full shadow-lg hover:scale-105 transition"
+          className="fixed bottom-28 right-11 z-50 rounded-full border border-cyan-200/70 bg-cyan-600 px-4 py-3 text-white shadow-lg shadow-cyan-900/30 transition hover:scale-105 hover:bg-cyan-500 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-cyan-300 focus-visible:ring-offset-2 focus-visible:ring-offset-white dark:border-cyan-300/30 dark:bg-cyan-500 dark:hover:bg-cyan-400 dark:focus-visible:ring-offset-[#0a0e27]"
           aria-label="Back to Top"
         >
           ↑

tests/test_back_to_top_styles.py

@@ -0,0 +1,14 @@
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def test_back_to_top_button_uses_high_contrast_theme_safe_styles():
+    source = (REPO_ROOT / "src/components/BackToTop.jsx").read_text(encoding="utf-8")
+
+    assert "bg-cyan-600" in source
+    assert "hover:bg-cyan-500" in source
+    assert "dark:bg-cyan-500" in source
+    assert "dark:hover:bg-cyan-400" in source
+    assert "focus-visible:ring-cyan-300" in source