refactor(cs): enforce space after NOT operator in code standards

- Add Generic.Formatting.SpaceAfterNot rule to phpcs.xml
- Update theme.php to use spaced NOT operator syntax

Author: Sébastien HEYD

phpcs.xml

@@ -116,6 +116,7 @@
     <rule ref="Squiz.WhiteSpace.ScopeClosingBrace"/>
     <rule ref="Squiz.WhiteSpace.ScopeKeywordSpacing"/>
     <rule ref="Squiz.WhiteSpace.SemicolonSpacing"/>
+    <rule ref="Generic.Formatting.SpaceAfterNot"/>
     <rule ref="Zend.Files.ClosingTag"/>
 
     <file>src</file>

src/config/theme.php

@@ -5,7 +5,7 @@
 
 // Check if theme exists
 $themePath = __DIR__.'/themes/'.$selectedTheme.'.php';
-if (!file_exists($themePath)) {
+if (! file_exists($themePath)) {
     $selectedTheme = 'default';
     $themePath = __DIR__.'/themes/default.php';
 }