as font seems to be too small on device, I try out sp(24 instead of app.font_size)

.idea/workspace.xml

@@ -4,7 +4,7 @@
     <option name="autoReloadType" value="SELECTIVE" />
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="3745efb7-4727-49d7-a776-8798725fc688" name="Changes" comment="Now with image!">
+    <list default="true" id="3745efb7-4727-49d7-a776-8798725fc688" name="Changes" comment="Adds icon">
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
@@ -15,6 +15,9 @@
   <component name="Git.Settings">
     <option name="RECENT_GIT_ROOT_PATH" value="$PROJECT_DIR$" />
   </component>
+  <component name="MarkdownSettingsMigration">
+    <option name="stateVersion" value="1" />
+  </component>
   <component name="ProblemsViewState">
     <option name="selectedTabId" value="CurrentFile" />
   </component>
@@ -100,13 +103,31 @@
       <option name="project" value="LOCAL" />
       <updated>1714488962508</updated>
     </task>
-    <option name="localTasksCounter" value="4" />
+    <task id="LOCAL-00004" summary="Adds Readme">
+      <option name="closed" value="true" />
+      <created>1714509795544</created>
+      <option name="number" value="00004" />
+      <option name="presentableId" value="LOCAL-00004" />
+      <option name="project" value="LOCAL" />
+      <updated>1714509795544</updated>
+    </task>
+    <task id="LOCAL-00005" summary="Adds icon">
+      <option name="closed" value="true" />
+      <created>1714510610620</created>
+      <option name="number" value="00005" />
+      <option name="presentableId" value="LOCAL-00005" />
+      <option name="project" value="LOCAL" />
+      <updated>1714510610620</updated>
+    </task>
+    <option name="localTasksCounter" value="6" />
     <servers />
   </component>
   <component name="VcsManagerConfiguration">
     <MESSAGE value="It’s running! Yeah!" />
     <MESSAGE value="Still running… in multiple files, with global font_size…" />
     <MESSAGE value="Now with image!" />
-    <option name="LAST_COMMIT_MESSAGE" value="Now with image!" />
+    <MESSAGE value="Adds Readme" />
+    <MESSAGE value="Adds icon" />
+    <option name="LAST_COMMIT_MESSAGE" value="Adds icon" />
   </component>
 </project>

main.py

@@ -4,7 +4,6 @@ from rannum import RanNum
 
 
 class RanKivApp(App):
-    font_size = NumericProperty(24)
 
     def build(self):
         self.ran_num = RanNum()

rankiv.kv

@@ -7,7 +7,7 @@
             id: upper_boundary_input
             multiline: False
             halign: 'center'
-            font_size: app.font_size
+            font_size: sp(24)
             size_hint: .8, None
             height: 42
             hint_text: 'Gib eine Zahl ein!'
@@ -17,7 +17,7 @@
             size_hint: None, .2
         Button:
             text: 'Generate!'
-            font_size: app.font_size
+            font_size: sp(24)
             size_hint: .5, None
             pos_hint: {'x': .25}
             on_release: app.ran_num.generate_random_number()
@@ -25,4 +25,4 @@
             id: output
             text: 'Meine Zufallszahl'
             halign: 'center'
-            font_size: app.font_size
+            font_size: sp(24)