diff --git a/src/css/custom.css b/src/css/custom.css
index 4d8e204b..a4038f8e 100644
--- a/src/css/custom.css
+++ b/src/css/custom.css
@@ -2092,24 +2092,58 @@ html[data-theme="dark"] {
   flex: 0 0 280px !important;
   width: 280px !important;
   max-width: 280px !important;
-  padding-left: 1rem !important;
+  padding-left: 0 !important;
   padding-right: 0.5rem !important;
+  border-left: 2px solid var(--ifm-toc-border-color) !important;
+  align-self: stretch !important;
+}
+
+/* TOC inner wrapper — sticky */
+.blog-wrapper .col.col--2 > div {
+  position: sticky !important;
+  top: calc(var(--ifm-navbar-height, 60px) + 1rem) !important;
+  max-height: calc(100vh - var(--ifm-navbar-height, 60px) - 2rem) !important;
+  overflow-y: auto !important;
+}
+
+/* TOC list — vertical left border track */
+.blog-wrapper .table-of-contents {
+  border-left: 2px solid var(--ifm-toc-border-color) !important;
+  padding-left: 0 !important;
+  margin-left: 0.75rem !important;
 }
 
 /* TOC links */
 .blog-wrapper .table-of-contents__link {
   white-space: normal !important;
   word-break: break-word !important;
-  line-height: 1.4 !important;
+  line-height: 1.5 !important;
   display: block !important;
-  padding: 2px 0 !important;
+  padding: 4px 0 4px 12px !important;
+  margin-left: -2px !important;
   font-size: 0.85rem !important;
+  color: var(--ifm-color-emphasis-600) !important;
+  border-left: 2px solid transparent !important;
+  transition: color 0.2s ease, border-color 0.2s ease !important;
+}
+
+.blog-wrapper .table-of-contents__link:hover {
+  color: var(--ifm-color-primary) !important;
+  text-decoration: none !important;
 }
 
 .blog-wrapper .table-of-contents li>.table-of-contents__link {
   font-size: 0.875rem !important;
 }
 
+/* Active TOC link — highlight current section with left border indicator */
+.blog-wrapper .table-of-contents__link--active,
+.blog-wrapper .table-of-contents__link--active:hover {
+  color: var(--ifm-color-primary) !important;
+  font-weight: 600 !important;
+  border-left: 2px solid var(--ifm-color-primary) !important;
+}
+
 .blog-post-page .markdown {
   max-width: 100% !important;
   line-height: 1.8 !important;
@@ -2182,6 +2216,17 @@ html[data-theme="dark"] {
     align-items: flex-start !important;
   }
 
+  /* Remove the vertical separator and sticky on mobile */
+  .blog-wrapper .container>.row>.col.col--2 {
+    border-left: none !important;
+  }
+
+  .blog-wrapper .col.col--2 > div {
+    position: static !important;
+    max-height: none !important;
+    overflow-y: visible !important;
+  }
+
   .blog-wrapper .container>.row>.col:not(.col--2):not(.col--3)>* {
     max-width: 100% !important;
   }