Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/12/21/customizing-the-flex-combobox-controls-text-input-styles/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Style>
        ComboBox {
            textInputStyleName: myCustomTextInputStyleName;
        }

        .myCustomTextInputStyleName {
            color: red;
            fontStyle: italic;
            fontWeight: bold;
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="editable:">
                <mx:CheckBox id="checkBox"
                        selected="false" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:ComboBox id="comboBox"
            editable="{checkBox.selected}"
            creationComplete="comboBox.open();">
        <mx:dataProvider>
            <mx:Array>
                <mx:Object label="One" />
                <mx:Object label="Two" />
                <mx:Object label="Three" />
                <mx:Object label="Four" />
                <mx:Object label="Five" />
                <mx:Object label="Six" />
                <mx:Object label="Seven" />
            </mx:Array>
        </mx:dataProvider>
    </mx:ComboBox>

</mx:Application>

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