docs: add README and dex custom theme

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dextpl/login.html

@@ -0,0 +1,19 @@
+{{ template "header.html" . }}
+
+<div class="theme-panel">
+  <h2 class="theme-heading">Log in to {{ issuer }} </h2>
+  <div>
+    {{ range $c := .Connectors }}
+      <div class="theme-form-row">
+        <a href="{{ $c.URL }}" target="_self">
+          <button class="dex-btn theme-btn-provider">
+            <span class="dex-btn-icon dex-btn-icon--{{ $c.Type }}"></span>
+            <span class="dex-btn-text">Log in with {{ $c.Name }}</span>
+          </button>
+        </a>
+      </div>
+    {{ end }}
+  </div>
+</div>
+
+{{ template "footer.html" . }}