How to remove Bottom Border Textbox | UWP

Hi @Anderson Rodrigues Cavalcante ,

Welcome to Microsoft Q&A!

The default template of Textbox can be found in generic.xaml in: C:\Program Files (x86)\Windows Kits\10\DesignTime\CommonConfiguration\Neutral\UAP\{version}\Generic.

Find the resource TextControlBorderBrushFocused and modify its value with a new SolidColorBrush.

However, if my SO is in High Contrast mode, I'd like to keep these native borders

In HighContrast mode, TextControlBorderBrushFocused"ResourceKey" still use default SystemControlHighlightAccentBrush.

<TextBox Text="TextBox" >
    <TextBox.Resources>
        <ResourceDictionary>
            <ResourceDictionary.ThemeDictionaries>
                <ResourceDictionary x:Key="Dark">
                    <SolidColorBrush x:Key="TextBorderTransparentBrush" Color="Red" />
                    <StaticResource x:Key="TextControlBorderBrushFocused" ResourceKey="TextBorderTransparentBrush" />
                </ResourceDictionary>
                <ResourceDictionary x:Key="Light">
                    <SolidColorBrush x:Key="TextBorderTransparentBrush" Color="Green" />
                    <StaticResource x:Key="TextControlBorderBrushFocused" ResourceKey="TextBorderTransparentBrush" />
                </ResourceDictionary>
                <ResourceDictionary x:Key="HighContrast">
                    <StaticResource x:Key="TextControlBorderBrushFocused" ResourceKey="SystemControlHighlightAccentBrush" />
                </ResourceDictionary>
            </ResourceDictionary.ThemeDictionaries>
        </ResourceDictionary>
    </TextBox.Resources>
</TextBox>

Thank you.

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