[fix](trx-frontend-http): style theme toggle button as the opposite theme

When in dark mode the button has a light appearance; when in light mode
it has a dark appearance. This makes the button a preview of what
clicking will switch to, rather than mirroring the current theme.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -490,6 +490,17 @@ small { color: var(--text-muted); }
   font-size: 0.78rem;
   white-space: nowrap;
 }
+/* Theme toggle: always styled as the opposite theme so it previews what clicking will do */
+#theme-toggle {
+  background: #dde3ed;
+  color: #1a2336;
+  border-color: #a0aec0;
+}
+[data-theme="light"] #theme-toggle {
+  background: #1e2a3a;
+  color: #e7edf9;
+  border-color: #3a5274;
+}
 .header-style-pick {
   display: flex;
   align-items: center;