refactor: use tailwind custom colors for themes

Use tailwind v4 @theme block so we can use bg-layer-0 instead of
bg-[--layer-0] for theme colors.

app/src/lib/graph-interface/node/NodeHTML.svelte

@@ -71,22 +71,22 @@
     user-select: none !important;
     cursor: pointer;
     width: 200px;
-    color: var(--text-color);
+    color: var(--color-text);
     transform: translate3d(var(--nx), var(--ny), 0);
     z-index: 1;
     opacity: calc((var(--cz) - 2.5) / 3.5);
     font-weight: 300;
-    --stroke: var(--outline);
+    --stroke: var(--color-outline);
     --stroke-width: 2px;
   }
 
   .node.active {
-    --stroke: var(--active);
+    --stroke: var(--color-active);
     --stroke-width: 2px;
   }
 
   .node.selected {
-    --stroke: var(--selected);
+    --stroke: var(--color-selected);
     --stroke-width: 2px;
   }
 </style>

app/src/lib/graph-interface/node/NodeHeader.svelte

@@ -109,7 +109,7 @@
   svg path {
     stroke-width: 0.2px;
     transition: d 0.3s ease, fill 0.3s ease;
-    fill: var(--layer-2);
+    fill: var(--color-layer-2);
     stroke: var(--stroke);
     stroke-width: var(--stroke-width);
     d: var(--path);

app/src/lib/graph-interface/node/NodeParameter.svelte

@@ -175,7 +175,7 @@
 
   svg path {
     transition: d 0.3s ease, fill 0.3s ease;
-    fill: var(--layer-1);
+    fill: var(--color-layer-1);
     stroke: var(--stroke);
     stroke-width: var(--stroke-width);
     d: var(--path);