[chore]: Use theme color variables instead of hardcoded values #661

Replace hardcoded colors in CSS files and templates with the CSS variables introduced in openwisp-utils#516 to ensure unified theming and simplify future palette updates.

Closes #661

Author: shivsubh

openwisp_radius/integrations/monitoring/static/radius-monitoring/css/device-change.css

@@ -10,7 +10,7 @@
   height: 4em;
 }
 #radius-session-tbody strong {
-  color: green;
+  color: var(--ow-color-success, green);
 }
 #radius-session-tbody tr td p {
   white-space: nowrap;

openwisp_radius/static/openwisp-radius/css/mode-switcher.css

@@ -6,5 +6,5 @@
 .form-row.field-user label,
 .form-row.field-username label {
   font-weight: bold;
-  color: #333;
+  color: var(--ow-color-fg-darker, #333);
 }

openwisp_radius/static/openwisp-radius/css/radiusbatch.css

@@ -2,5 +2,5 @@
 .field-prefix label,
 .field-number_of_users label {
   font-weight: bold;
-  color: #000;
+  color: var(--ow-color-black, #000);
 }

openwisp_radius/templates/openwisp-radius/prefix_pdf.html

@@ -10,7 +10,7 @@
         table { width: 100%; border-collapse: collapse }
         table th { font-weight: bold }
         table th, table td {
-            border: 1px solid #ccc;
+            border: 1px solid var(--ow-color-fg-medium, #ccc);
             padding: 10px;
         }
     </style>