fix: improve dark mode contrast for wells and balance display

- Change dark mode --color-well from #0a0a0a to #111111 for visible
  contrast against #000000 background
- Add explicit text-fg class to balance display element to ensure
  white text in dark mode

src/popup/index.html

@@ -185,7 +185,7 @@
                 <!-- active address headline -->
                 <div
                     id="total-value"
-                    class="text-2xl font-bold min-h-[2rem]"
+                    class="text-2xl font-bold min-h-[2rem] text-fg"
                 ></div>
                 <div
                     id="total-value-sub"