Some changes...

app/src/main/java/meow/sweetbread/lincalc/MainActivity.kt

@@ -184,7 +184,8 @@ class MainActivity : AppCompatActivity() {
             }
             else if (tokensType.containsKey(chr.toString())) {
                 if (tokens.isNotEmpty()) {
-                    if ((tokensType[chr.toString()]!! == Tokens.BracketOpen) and (tokens.last().type == Tokens.Number))
+                    if ((tokensType[chr.toString()]!! == Tokens.BracketOpen) and
+                        (tokens.last().type in arrayOf(Tokens.Number, Tokens.Variable)))
                         tokens += Token(Tokens.Operations, "*")
                     if ((chr == '-') and (tokens.last().type != Tokens.Number))
                         tokens += Token(Tokens.Number, 0.toDouble())

app/src/main/res/drawable-night/button.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:tools="http://schemas.android.com/tools" xmlns:android="http://schemas.android.com/apk/res/android" tools:ignore="MissingDefaultResource">
+<!--    <item-->
+<!--        android:state_enabled="false"-->
+<!--        android:drawable="@drawable/button_disabled" />-->
+<!--    <item-->
+<!--        android:state_pressed="true"-->
+<!--        android:state_enabled="true"-->
+<!--        android:drawable="@drawable/button_pressed" />-->
+<!--    <item-->
+<!--        android:state_focused="true"-->
+<!--        android:state_enabled="true"-->
+<!--        android:drawable="@drawable/button_focused" />-->
+    <item
+        android:state_enabled="true"
+        android:drawable="@drawable/button_enabled" />
+</selector>

app/src/main/res/drawable-night/button_enabled.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <solid android:color="@color/design_default_color_primary_dark">
+
+    </solid>
+</shape>

app/src/main/res/layout/activity_main.xml

@@ -6,6 +6,17 @@
     android:layout_height="match_parent"
     tools:context=".MainActivity">
 
+    <ImageView
+        android:id="@+id/imageView2"
+        android:layout_width="0dp"
+        android:layout_height="0dp"
+        android:scaleType="centerCrop"
+        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toTopOf="@+id/input"
+        app:srcCompat="@drawable/background" />
+
     <TextView
         android:id="@+id/input"
         android:layout_width="0dp"