Give select same border colour as other form elements

Towards #163

assets/css/base.css

@@ -313,6 +313,7 @@ input[type=radio] + label {
     background-repeat: no-repeat, repeat;
     background-position: right 0.75rem top 50%, 0 0;
     background-size: 0.8em 0.8em, 100%;
+    border-color: var(--border);
     border-radius: 0.5em;
     box-shadow: 0 0.05em 0 0.05em var(--shadow);
     color: var(--text);