Theme Customization

This guide shows safe customization layers for TreeDataGrid theme behavior in XAML.

Level 1: Override Built-In Resource Keys

For most visual tweaks, override TreeDataGrid brush keys instead of replacing control templates.

<Application.Resources>
  <SolidColorBrush x:Key="TreeDataGridGridLinesBrush" Color="#60FFB000"/>
  <SolidColorBrush x:Key="TreeDataGridSelectedCellBackgroundBrush" Color="#66984AFB"/>
  <SolidColorBrush x:Key="TreeDataGridHeaderBackgroundPointerOverBrush" Color="#22007ACC"/>
</Application.Resources>

This is low-risk and survives future control-template changes better than full template replacement.

Level 2: Per-Grid Styling with Selectors

Use TreeDataGrid.Styles to apply local, scenario-specific styles:

<TreeDataGrid>
  <TreeDataGrid.Styles>
    <Style Selector="TreeDataGrid TreeDataGridRow:nth-child(2n)">
      <Setter Property="Background" Value="#12808080"/>
    </Style>
    <Style Selector="TreeDataGrid TreeDataGridColumnHeader:nth-last-child(1)">
      <Setter Property="TextBlock.FontWeight" Value="Bold"/>
    </Style>
  </TreeDataGrid.Styles>
</TreeDataGrid>

Level 3: Replace Specific ControlTheme Keys

Advanced customization can replace ControlTheme entries from Generic.axaml (for example, expander chevron theme key TreeDataGridExpandCollapseChevron).

Prefer replacing one focused theme key rather than rewriting all TreeDataGrid control templates.

For deeper guidance:

use ControlTheme Overrides with BasedOn when you want to inherit shipped templates
use ControlTheme Full Replacement when you need a new visual tree

Theme Override Order

Recommended setup:

Include TreeDataGrid theme file in Application.Styles.
Define overrides in the intended scope (Application.Resources for global, TreeDataGrid.Resources/TreeDataGrid.Styles for local behavior).
Validate both Light and Dark variants.

Troubleshooting

Override works in Light but not Dark Cause: only one theme dictionary branch is overridden. Fix: define overrides for both variants or use variant-aware resources.
Custom selector does not apply Cause: selector does not match actual visual tree/control type. Fix: inspect runtime tree and adjust selector specificity.
Global override does not affect one grid Cause: grid-local resource/style definitions are shadowing app-level values. Fix: inspect nearest resource scope and remove or adjust local overrides.