Since we're changing this line, we may as well improve it. 

HashMap.get() can take null or empty keys, so there is no need to check the key first. Something like this would be better:
```java
AnkiFont font = customFontsMap.get(defaultFont);
if (font != null) {
    mCustomDefaultFontCss = "BODY .question, BODY .answer { " + font.getCSS() + " }\n";
}
```
This also avoids hashing the key twice.