a RadRibbonBar. The latter is now obsolete and has been upgraded into the former)
will bring you right to a tutorial on how to change what you're looking for. In addition, I have pasted the code below (inside a UserControlResources tag) for you to get started. Read the documentatin to familiarize yourself with the template.
That link I gave you helps step you through the process to get into the template. For your purposes we are going to focus on one part. Look down the visual tree and notice the
Expand this item and you'll see a border, MouseOverVisual, SelectedVisual, and HeaderElement.
Here is the place you want make your changes. To give you a head start, I've pasted some code below for you to use. It is the entire UserControl.Resources and root grid of a RadRibbonView control with a template ready for you to alter. The other tabs are there see compare against.
<UserControl.Resources>
<LinearGradientBrush x:Key="TabStripTabDividerBrush" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#4c4c4c" Offset="0"/>
<GradientStop Color="#3b3b3b" Offset="0.3"/>
<GradientStop Color="#363636" Offset="1"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="TabMouseOverOuterBorder" Color="#FF848484"/>
<SolidColorBrush x:Key="TabMouseOverInnerBorder" Color="#FFFFFFFF"/>
<LinearGradientBrush x:Key="TabMouseOverBackground" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFFFFBA3" Offset="0"/>
<GradientStop Color="#FFFFFBDA" Offset="1"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="TabSelectedOuterBorder" Color="#FF848484"/>
<SolidColorBrush x:Key="TabSelectedInnerBorder" Color="#FFFFFFFF"/>
<LinearGradientBrush x:Key="TabSelectedBackground" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#FFFAFAFA" Offset="1"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="RibbonTabOuterBorderBrush" Color="#FF848484"/>
<LinearGradientBrush x:Key="RibbonTabBackgroundBrush" EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="#FFFEFEFE"/>
<GradientStop Color="#FFD4D4D4" Offset="1"/>
</LinearGradientBrush>
<SolidColorBrush x:Key="RibbonTabInnerBorderBrush" Color="Transparent"/>
<telerik:Office_BlackTheme x:Key="Theme"/>
<CornerRadius x:Key="RibbonTabInnerCornerRadius">2.5</CornerRadius>
<CornerRadius x:Key="RibbonTabOuterCornerRadius">3.5</CornerRadius>
<Style x:Key="RadRibbonTabStyle" TargetType="telerik:RadRibbonTab">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1,0,0,0"/>
<Setter Property="Foreground" Value="#000000"/>
<Setter Property="BorderBrush" Value="{StaticResource TabStripTabDividerBrush}"/>
<Setter Property="IsTabStop" Value="False"/>
<Setter Property="Margin" Value="2 2 2 0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="telerik:RadRibbonTab">
<Grid x:Name="RootPanel">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="MouseOverVisual">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="MouseOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="MouseOverVisual">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Selected">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="SelectedVisual">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="NotSelected">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="SelectedVisual">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="EnabledStates">
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To="0.5" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="HeaderElement"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Enabled"/>
</VisualStateGroup>
<VisualStateGroup x:Name="ContextualTabStates">
<VisualState x:Name="NotContextualTab">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="HeaderPanel">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ActiveContextualTab">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="HeaderPanel">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="InactiveContextualTab">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="HeaderPanel">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="TabSeparator" Fill="{TemplateBinding BorderBrush}" HorizontalAlignment="Right" Margin="0,0,-3,0" Visibility="Collapsed" VerticalAlignment="Stretch" Width="1"/>
<Grid x:Name="HeaderPanel" Visibility="{Binding HeaderVisibility, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}">
<Border Background="{TemplateBinding Background}"/>
<Border x:Name="MouseOverVisual" BorderBrush="{StaticResource TabMouseOverOuterBorder}" BorderThickness="1 1 1 0" CornerRadius="3 3 0 0" Visibility="Collapsed">
<Border BorderBrush="{StaticResource TabMouseOverInnerBorder}" BorderThickness="1 1 1 0" Background="{StaticResource TabMouseOverBackground}" CornerRadius="2 2 0 0"/>
</Border>
<Border x:Name="SelectedVisual" BorderBrush="{StaticResource TabSelectedOuterBorder}" BorderThickness="1 1 1 0" CornerRadius="3 3 0 0" Visibility="Collapsed">
<Border BorderBrush="{StaticResource TabSelectedInnerBorder}" BorderThickness="1 1 1 0" Background="{StaticResource TabSelectedBackground}" CornerRadius="2 2 0 0"/>
</Border>
<Telerik_Windows_Controls_Primitives:TabItemContentPresenter x:Name="HeaderElement" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Foreground="{TemplateBinding Foreground}" HorizontalAlignment="Center" HorizontalContentAlignment="Center" IsTabStop="False" Margin="4,2" VerticalAlignment="Center" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
<Grid x:Name="ContentPanel" Visibility="{TemplateBinding Visibility}" Width="0">
<Border x:Name="OuterBorder" BorderBrush="{StaticResource RibbonTabOuterBorderBrush}" BorderThickness="1" Background="{StaticResource RibbonTabBackgroundBrush}" CornerRadius="{StaticResource RibbonTabOuterCornerRadius}">
<Border x:Name="InnerBorder" BorderBrush="{StaticResource RibbonTabInnerBorderBrush}" BorderThickness="1" CornerRadius="{StaticResource RibbonTabInnerCornerRadius}">
<Telerik_Windows_Controls_RibbonView_Primitives:RibbonScrollViewer x:Name="TabItemsScrollViewer" telerik:StyleManager.Theme="{StaticResource Theme}">
<ItemsPresenter HorizontalAlignment="Left"/>
</Telerik_Windows_Controls_RibbonView_Primitives:RibbonScrollViewer>
</Border>
</Border>
</Grid>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<Telerik_Windows_Controls_RibbonView_Primitives:RibbonGroupsPanel/>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
</Style>
</UserControl.Resources>
<Grid x:Name="LayoutRoot" Background="White">
<telerik:RadRibbonView d:LayoutOverrides="Width">
<telerik:RadRibbonView.ContextualGroups>
<telerik:RadRibbonContextualGroup x:Name="ContextualGroup1"
Header="Group 1"
IsActive="False"
Color="Coral" />
<telerik:RadRibbonContextualGroup x:Name="ContextualGroup2"
Header="Group 2"
IsActive="False"
Color="Orchid" />
</telerik:RadRibbonView.ContextualGroups>
<telerik:RadRibbonTab x:Name="ThisTabIsTheOneYouWant" Header="RibbonTab" Style="{StaticResource RadRibbonTabStyle}">
<telerik:RadRibbonGroup Header="RibbonGroup"/>
</telerik:RadRibbonTab>
<telerik:RadRibbonTab x:Name="unaltered_Tab" ContextualGroupName="ContextualGroup1" Header="ContextualTab 1.1" />
</telerik:RadRibbonView>
</Grid>