diff --git a/src/Spout/Writer/XLSX/Manager/OptionsManager.php b/src/Spout/Writer/XLSX/Manager/OptionsManager.php
index 53718bc..0c08b5a 100644
--- a/src/Spout/Writer/XLSX/Manager/OptionsManager.php
+++ b/src/Spout/Writer/XLSX/Manager/OptionsManager.php
@@ -14,7 +14,7 @@ class OptionsManager extends OptionsManagerAbstract
 {
     /** Default style font values */
     const DEFAULT_FONT_SIZE = 12;
-    const DEFAULT_FONT_NAME = 'Calibri';
+    const DEFAULT_FONT_NAME = 'Arial';
 
     /** @var StyleBuilder Style builder */
     protected $styleBuilder;
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
index 0a72ba9..4a608ba 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
@@ -225,7 +225,7 @@ EOD;
             $styleId = $style->getId();
             $fillId = $this->getFillIdForStyleId($styleId);
             $borderId = $this->getBorderIdForStyleId($styleId);
-            $numberFormatId = !empty($style->getNumberFormat()) ? $style->getNumberFormat()->getId() : 0;
+            $numberFormatId = $this->getNumberFormatIdForStyleId($styleId);
 
             $content .= '<xf numFmtId="'.$numberFormatId.'" fontId="' . $styleId . '" fillId="' . $fillId . '" borderId="' . $borderId . '" xfId="0"';
 
@@ -282,6 +282,22 @@ EOD;
         return $isDefaultStyle ? 0 : ($this->styleRegistry->getBorderIdForStyleId($styleId) ?: 0);
     }
 
+    /**
+     * Returns the fill ID associated to the given style ID.
+     * For the default style, we don't a border.
+     *
+     * @param int $styleId
+     * @return int
+     */
+    private function getNumberFormatIdForStyleId($styleId)
+    {
+        // For the default style (ID = 0), we don't want to override the border.
+        // Otherwise all cells of the spreadsheet will have a border.
+        $isDefaultStyle = ($styleId === 0);
+
+        return $isDefaultStyle ? 0 : ($this->styleRegistry->getNumberFormatIdForStyleId($styleId) ?: 0);
+    }
+
     /**
      * Returns the content of the "<cellStyles>" section.
      *