fix(markdown): invert TikZ SVGs on dark themes for readability

src/styles/app.css

@@ -202,6 +202,18 @@ pre.tikz svg {
   height: auto;
 }
 
+/* TikZ SVGs are monochrome black; invert them on dark themes so they
+   remain readable on the transparent background. */
+[data-theme="dracula"] pre.tikz svg {
+  filter: invert(1) hue-rotate(180deg);
+}
+
+@media (prefers-color-scheme: dark) {
+  :root:not([data-theme="light"]) pre.tikz svg {
+    filter: invert(1) hue-rotate(180deg);
+  }
+}
+
 .tikz-loading {
   opacity: 0.6;
   font-style: italic;