Enhance button styling and layout in Song Submission screen for improved UI experience

client/src/common/styles/main.sass

@@ -416,6 +416,7 @@ h1, h2, h3
     justify-content: center
     align-items: center
     cursor: pointer
+    color: white
     transition: all 0.2s ease
     
     &:hover

client/src/components/SongSubmissionScreen.jsx

@@ -352,11 +352,15 @@ const SongSubmissionScreen = () => {
         
         {canSubmitMoreSongs && !isFormVisible && !isReady && (
           <button
-            className="add-song-btn"
+            className="btn add-song-btn"
             onClick={() => setIsFormVisible(true)}
           >
-            <FontAwesomeIcon icon={faPlus} />
+            <FontAwesomeIcon icon={faPlus} className="icon-margin" />
             <span>Lied hinzufügen</span>
+            <span className="pixel-corner tl"></span>
+            <span className="pixel-corner tr"></span>
+            <span className="pixel-corner bl"></span>
+            <span className="pixel-corner br"></span>
           </button>
         )}
       </div>