How to customize Skin Gallery

1. REMOVE (HIDE) A SPECIFIC SKIN 
Traverse through the gallery group collection, then through its gallery item collection and hide a corresponding item:

private void InitRibbonSkinGallery() {
            SkinHelper.InitSkinGallery(skinGalleryBarItem);
}

string[] skinsToHide = {"Black","Blue","Seven","Sharp" }; // populate with names of unnecessary skins
private void HideSkins(string[] skinsToHide) {
            for(var i = 0; i < skinGalleryBarItem.Gallery.Groups.Count; i++) {
                var group = skinGalleryBarItem.Gallery.Groups[i];
                if(group == null) {
                    continue;
                }
                for(var j = 0; j < group.Items.Count; j++) {
                    var item = group.Items[j];
                    if(item == null) {
                        continue;
                    }
                    foreach(var skin in skinsToHide) {
                        if(String.Equals(item.Caption, skin)) {
                            item.Visible = false;
                        }
                    }
                }
            }
}

Private Sub InitRibbonSkinGallery()
            SkinHelper.InitSkinGallery(skinGalleryBarItem)
End Sub

Private skinsToHide() As String = {"Black","Blue","Seven","Sharp" } 'populate with names of unnecessary skins
Private Sub HideSkins(ByVal skinsToHide() As String)
            For i = 0 To skinGalleryBarItem.Gallery.Groups.Count - 1
                Dim group = skinGalleryBarItem.Gallery.Groups(i)
                If group Is Nothing Then
                    Continue For
                End If
                For j = 0 To group.Items.Count - 1
                    Dim item = group.Items(j)
                    If item Is Nothing Then
                        Continue For
                    End If
                    For Each skin In skinsToHide
                            If String.Equals(item.Caption, skin) Then
                            item.Visible = False
                        End If
                    Next skin
                Next j
            Next i
End Sub

This is discussed in the How to remove certain skins from the bonus skins collection  ticket.

2. REMOVE  A SPECIFIC SKIN GROUP 
Remove a required group from the collection:

string skinGroup = "Standard Skins";
RemoveSkinGroups(skinGroup);

void RemoveSkinGroups(string skinGroup) {
            skinGalleryBarItem.Gallery.Groups.Remove(skinGalleryBarItem.Gallery.Groups.OfType<GalleryItemGroup>().First(x => String.Equals(x.Caption, skinGroup)));                       
}

Private skinGroup As String = "Standard Skins"
RemoveSkinGroups(skinGroup)

void RemoveSkinGroups(String skinGroup)
            skinGalleryBarItem.Gallery.Groups.Remove(skinGalleryBarItem.Gallery.Groups.OfType(Of GalleryItemGroup)().First(Function(x) String.Equals(x.Caption, skinGroup)))

This issue is discussed in the How to remove the "Theme Skin" skin group from the In-Ribbon gallery populated with available skins  thread.

3. REMOVE GROUPING 

Fill the In-Ribbon and In-Dropdown gallery with required skins manually. 
To obtain all available skins in your project, use the SkinManager.Skins property. To populate the In-Dropdown gallery, handle theRibbonGalleryBarItem.GalleryInitDropDownGallery event:

DevExpress.XtraBars.RibbonGalleryBarItem skinGalleryBarItem;
SkinContainerCollection skins;

void InitSkinGallery() {
            //SkinHelper.InitSkinGallery(skinGalleryBarItem, true);
            skins = SkinManager.Default.Skins;

            for (int i = 0; i < 14; i++) {
                int index = rgbiSkins.Gallery.Groups[0].Items.Add(new GalleryItem());
                GalleryItem item = skinGalleryBarItem.Gallery.Groups[0].Items[index];
                item.Description = skins[index].SkinName;
                item.Image = galleryImageCollection.Images[i];               
            }
}

private void skinGalleryBarItem_GalleryInitDropDownGallery(object sender, InplaceGalleryEventArgs e) {
            e.PopupGallery.AllowHoverImages = false;
            e.PopupGallery.ItemClick += new GalleryItemClickEventHandler(PopupGallery_ItemClick);
            for (int i = 0; i < e.PopupGallery.Groups[0].Items.Count; i++) {
                GalleryItem item = e.PopupGallery.Groups[0].Items[i];
                item.Description = skins[i].SkinName;
                item.Caption = skins[i].SkinName;
            }
}
void PopupGallery_ItemClick(object sender, GalleryItemClickEventArgs e) {
            defaultBarAndDockingController1.Controller.LookAndFeel.SkinName = e.Item.Description;
}

Private skinGalleryBarItem As DevExpress.XtraBars.RibbonGalleryBarItem
Private skins As SkinContainerCollection

Private Sub InitSkinGallery()
            'SkinHelper.InitSkinGallery(skinGalleryBarItem, true);
            skins = SkinManager.Default.Skins

            For i As Integer = 0 To 13
                Dim index As Integer = rgbiSkins.Gallery.Groups(0).Items.Add(New GalleryItem())
                Dim item As GalleryItem = skinGalleryBarItem.Gallery.Groups(0).Items(index)
                item.Description = skins(index).SkinName
                item.Image = galleryImageCollection.Images(i)
            Next i
End Sub

Private Sub skinGalleryBarItem_GalleryInitDropDownGallery(ByVal sender As Object, ByVal e As InplaceGalleryEventArgs)
            e.PopupGallery.AllowHoverImages = False
            AddHandler e.PopupGallery.ItemClick, AddressOf PopupGallery_ItemClick
            For i As Integer = 0 To e.PopupGallery.Groups(0).Items.Count - 1
                Dim item As GalleryItem = e.PopupGallery.Groups(0).Items(i)
                item.Description = skins(i).SkinName
                item.Caption = skins(i).SkinName
            Next i
End Sub
Private Sub PopupGallery_ItemClick(ByVal sender As Object, ByVal e As GalleryItemClickEventArgs)
            defaultBarAndDockingController1.Controller.LookAndFeel.SkinName = e.Item.Description
End Sub

See Skin Gallery - How to remove skin grouping  ticket to learn more. 

4. CHANGE A SKIN NAME 
Traverse through the gallery group collection, then through its gallery item collection, obtain a required GalleryItem  and change the GalleryItem.Caption property for this purpose. 
See How to change/remove the DevExpress Style caption from the Skin Menu/Drop-down gallery  for more information.

5. CHANGE A SKIN ICON 

Traverse through the gallery group  collection, then through its gallery item  collection, obtain a required GalleryItem  and set the Image and HoverImage properties. 
See Skin Gallery - How to change a skin icon (image) and name (caption)  for more information.