CSS refactor: border-width variable, darker border palette, hover/padding cleanup

wwwroot/css/prototype.css

@@ -14,7 +14,7 @@ main {
 #sidebar {
   width: 260px;
   background: var(--f1-bg);
-  border-right: 1px solid var(--f2-border);
+  border-right: var(--border-width) solid var(--f1-border);
   display: flex;
   flex-direction: column;
   flex-shrink: 0;
@@ -23,7 +23,7 @@ main {
 
 .sidebar-header {
   padding: var(--s4);
-  border-bottom: 1px solid var(--f2-border);
+  border-bottom: var(--border-width) solid var(--f1-border);
   display: flex;
   align-items: center;
   justify-content: space-between;
@@ -53,9 +53,5 @@ main {
   white-space: nowrap;
   overflow: hidden;
   text-overflow: ellipsis;
-  border: 1px solid transparent;
-}
-
-.session-date {
-  color: var(--f3-border);
+  border: var(--border-width) solid transparent;
 }