Update predefined_themes.md

Author: gdt050579

docs/chapter-6/predefined_themes.md

@@ -2,11 +2,14 @@
 
 AppCUI comes with a set of predefined themes that can be used out of the box via the `Themes` enum. The following themes are available:
 
-1. **Default** - the default theme used by the library. 
+1. **Default** - the default theme used by the library.
+   
    <img src="img/default_theme.png" width=500/>
 
 2. **DarkGray** - a dark gray theme.
+
     <img src="img/darkgray_theme.png" width=500/>
 
 3. **Light** - a light theme.
-    <img src="img/light_theme.png" width=500/>
+   
+    <img src="img/light_theme.png" width=500/>