fab: use plus as icon

app/src/main/res/drawable/ic_add_black_24dp.xml

@@ -0,0 +1,9 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+  <path
+      android:pathData="M19,13h-6v6h-2v-6H5v-2h6V5h2v6h6v2z"
+      android:fillColor="#000000"/>
+</vector>

app/src/main/res/layouts/fragments/layout/fragment_timetable.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
-             xmlns:tools="http://schemas.android.com/tools"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    xmlns:tools="http://schemas.android.com/tools"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              tools:context=".fragments.TimeTableFragment"
@@ -24,15 +25,17 @@
     </androidx.swiperefreshlayout.widget.SwipeRefreshLayout>
 
     <com.google.android.material.floatingactionbutton.FloatingActionButton
-            android:id="@+id/faBtnAddSubject"
-            android:layout_width="wrap_content"
-            android:layout_height="wrap_content"
-            android:layout_gravity="bottom|center|end"
-            android:layout_marginEnd="11dp"
-            android:layout_marginBottom="11dp"
-            android:clickable="true"
-            android:elevation="7dp"
-            android:focusable="true"
-            android:src="@drawable/icon_custom_black"
-            android:visibility="visible" />
+        android:id="@+id/faBtnAddSubject"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_gravity="bottom|center|end"
+        android:layout_marginEnd="11dp"
+        android:layout_marginBottom="11dp"
+        android:clickable="true"
+        android:elevation="7dp"
+        android:focusable="true"
+        android:src="@drawable/ic_add_black_24dp"
+        android:visibility="visible"
+        app:backgroundTint="?colorAccent"
+        app:fabSize="auto" />
 </FrameLayout>