fix Illustration and IllustrationBlock alt color on dark mode (#5981)

Author: oxcened

src/components/MDX/MDXComponents.tsx

@@ -243,7 +243,7 @@ function Illustration({
           src={src}
           alt={alt}
           style={{maxHeight: 300}}
-          className="bg-white rounded-lg"
+          className="rounded-lg"
         />
         {caption ? (
           <figcaption className="text-center leading-tight mt-4">
@@ -275,7 +275,12 @@ function IllustrationBlock({
   const images = imageInfos.map((info, index) => (
     <figure key={index}>
       <div className="bg-white rounded-lg p-4 flex-1 flex xl:p-6 justify-center items-center my-4">
-        <img src={info.src} alt={info.alt} height={info.height} />
+        <img
+          className="text-primary"
+          src={info.src}
+          alt={info.alt}
+          height={info.height}
+        />
       </div>
       {info.caption ? (
         <figcaption className="text-secondary dark:text-secondary-dark text-center leading-tight mt-4">