fix(mermaid): Use dark theme if appropriate

Fixes: #4814

Signed-off-by: Jonas <jonas@freesources.org>

[skip ci]

Author: mejo-

src/nodes/CodeBlockView.vue

@@ -131,7 +131,9 @@ export default {
 		},
 	},
 	setup() {
+		const isDarkTheme = useIsDarkTheme()
 		return {
+			isDarkTheme,
 			/** The lazy loaded mermaid js module */
 			mermaid: null,
 		}
@@ -209,7 +211,10 @@ export default {
 				// lazy load mermaid on first real usage
 				if (this.mermaid === null) {
 					this.mermaid = (await import('mermaid')).default
-					this.mermaid.initialize({ startOnLoad: false })
+					this.mermaid.initialize({
+						startOnLoad: false,
+						theme: this.isDarkTheme ? 'dark' : 'default',
+					})
 				}
 				await this.mermaid.parse(textContent)