diff --git a/thumbnail/textcreator.cpp b/thumbnail/textcreator.cpp
--- a/thumbnail/textcreator.cpp
+++ b/thumbnail/textcreator.cpp
@@ -31,6 +31,7 @@
 
 #include <KSyntaxHighlighting/SyntaxHighlighter>
 #include <KSyntaxHighlighting/Theme>
+#include <KSyntaxHighlighting/Format>
 #include <KSyntaxHighlighting/Definition>
 #include <KDesktopFile>
 
@@ -140,9 +141,6 @@
                 }
             }
 
-            QColor bgColor = QColor ( 245, 245, 245 ); // light-grey background
-            m_pixmap.fill( bgColor );
-
             QPainter painter( &m_pixmap );
 
             QTextDocument textDocument(text);
@@ -167,6 +165,16 @@
             const auto highlightingTheme = m_highlightingRepository.defaultTheme(KSyntaxHighlighting::Repository::LightTheme);
             syntaxHighlighter.setTheme(highlightingTheme);
 
+            // apply background color from theme
+            m_pixmap.fill(highlightingTheme.editorColor(KSyntaxHighlighting::Theme::EditorColorRole::BackgroundColor));
+
+            if (!syntaxHighlighter.definition().isValid() ||
+                    (syntaxHighlighter.definition().includedDefinitions().isEmpty() && syntaxHighlighter.definition().formats().isEmpty())) {
+                // when no definition is found for the file, force syntax re-highlighting
+                // KSyntaxHighlighter does not highlight files with invalid definition by default (SyntaxHighlighter::setDefinition)
+                syntaxHighlighter.rehighlight();
+            }
+
             // draw page-in-page, with clipping as needed
             painter.translate(xborder, yborder);
             textDocument.drawContents(&painter, QRectF(QPointF(0, 0), canvasSize));