Develop Reference

Where does the look of wpf come from?

I just read the book on WPF from Thomas Claudius Huber. He stated, that all WPF controls are "lookless". They just get their look (and visual tree) from their ControlTemplate. That raises one question: Where does the look of WPF come from?
I mean: the Button has a ControlTemplate with some Borders and a ContentPresenter. Where do these two (Border and ContentPresenter) get their look from?
I already googled and found, that Border is a Decorator and sets its look in the OnRender-Method.
Is that the bottom line? Do all other elements which don't have a ControlTemplate define their look in the OnRender-Method?

Short answer: Yes. All visual elements that are not Controls and have a "look", define said look in their UIElement.OnRender method override.
Long answer: Controls don't use the OnRender method to define their looks. Instead, their "looks" are defined in Styles and Templates. When no Style or Template is defined explicitly in an application, WPF Controls simply use their default Styles and Templates from the current system Theme (for more info on Themes, check this MSDN article).
Let's just say that the Framework has its own Resource Dictionaries with default Styles for all built-in controls. For instance, here is the default ControlTemplate of the ComboBox: ComboBox Styles and Templates
That being said, there are several visual components that have their looks defined through code, usually through the OnRender override. They're not Controls; they're Decorators, Shapes and things like that. Things that do have a "look": Border, Rectangle, etc. But in the end, all Controls have look thanks to these elements because all ControlTemplates are made up of either these elements, or other Controls.
TextBlock, as Run, FlowDocument and other similar elements, are special elements created specifically for text renderization. They fall into a similar category than Shapes or Decorators, except they specialize on text rather than graphics. TextBlock, for instance, is not a Control, and defines its look on its OnRender method. Label, on the other hand, IS a Control; but if you check its Template, you'll see it ends up using a TextBlock to show the text.
There are other elements (like ContentPresenter, ItemsPresenter) that have no look whatsoever, not implicit, not by default, not by Styles or Templates. These are logic elements, that define the structure of the view. ContentPresenter, for instance, grabs the Content and ContentTemplate properties of a ContentControl, and makes sure that said Template is correctly rendered and bound to said data, so to speak. But they have no visual representation of their own.
Oh, and I almost forgot about Panels. Panels are not Controls, either, and they do have a look of their own. But similarly to Presenters, they're also logic elements that define how the other visual elements are visualized. More specifically, their layout.

represent data set in HubSection: Different styles for GridViewItems or access to GridView

I want to represent objects in HubSection, but they don't have to be presented in the same way. I have also two additional buttons: Add new session and Show all.
I wanted to use GridView, but I don't know how to use different styles depending of some the object's properties.
The second option is just create Grid and access to it from code according to this article.
Which option is recommended in this case, if first, how can I use different styles in GridView in Windows8.1 application?

ItemTemplate and ItemTemplateSelector is your friend when you want to achieve different appearance for items based on their properties
Example here http://www.geekchamp.com/articles/windows-8---winrt-datatemplateselector-change-the-color-of-listboxitem-depending-on-condition

Property to determine whether any descendant element has focus?

Suppose I have an element (in my case, a StackPanel) that contains several UI elements (in my case, lots of textboxes contained in various Grids contained in etc.etc. contained in the StackPanel).
I want to know whether any one of those textboxes has focus. (I want to bind this property to a View-Model property.) Is there a property for this? If not, what is the simplest way to bind to this kind of information, without having to first extract all the textboxes? (They’re generated by templates.)

You could use IsKeyboardFocusWithin. What kind of binding are you wanting to do to it? If it's something simple like you're wanting to change the background of the stackpanel if a textbox within has focus, you should be able to use this as a style trigger.

How is the Parent property of a FrameworkElement set in Silverlight?

I have written a custom Silverlight control based on Control. I have two DependencyProperties called Top and Bottom which both hold child controls for a specific layout display. I then use a ControlTemplate to arrange these two controls into a grid, placing one on the 0 row and the other on the 1 row. The problem I have is that I cannot seem to figure out how to get each child control's Parent property to point to my custom control. When I inspect each control at run-time, the Parent property of each is null.
This is a simple example, but I think you can see the general problem. I have a number of more complex controls that all share this problem. I know there is some magic I am missing. If a ContentControl's Content property is set to some child it is somehow setting that child's parent to itself.
Edit: A little more info
In WPF, one might use functions like AddVisualChild(), RemoveVisualChild(), AddLogicalChild(), RemoveLogicChild() to manage parent/child relationships, but these functions are not available in Silverlight.

After quite a bit of research I believe that this is not possible. I was able to recurse through the Visual Tree instead of the Logic Tree using the VisualTreeHelper to accomplish my ultimate goal.

The Parent property cannot be arbitrary, it reflects the real parent of the control for use when rendering.
From MSDN:
Parent may be a null reference (Nothing in Visual Basic) in cases where an element was instantiated, but is not attached to any logical tree that eventually connects to the page level root element, or the application object.
...
Changing an element's parent is typically only done through manipulation of collections, by using dedicated add or remove methods, or through setting content properties of elements.

Accessing TemplatePart without changing ControlTemplate?

i wonder if there is a way to access a control's templatepart from within c# for modifying the part (e.g. hiding, etc..). is it possible to get a reference to the part with pure c#?
i don't want to touch the controls template.
thanks
j.

It is possible, but its quite nasty.
On the Template there is a method called FindName, which needs two arguments: the name and the FrameworkElement that has the ControlTemplate as Template. Of course, you need to set the name of the element in the ControlTemplate...
Another more elegent solution is to use a Binding in the ControlTemplate to determine the visibility.. That way you do not need to do stuff in your code behind and you can keep it Xaml only...