Gnome 3.26 & compatiblites.
According to Gnome 3.26 features and change logs we have a transparent panel if desktop is shown (If any windows that is not maximized) and Fedora 27 will use of Gnome 3.26 but I am using Gnome 3.24 in Fedora 26 with a theme called Arc-dark so What will going to my Gnome if I update it to 3.26? A transparent panel? A dark panel like before? A compatibility with my theme and Gnome 3.26 changes?
That depends on a whole lot of things. If you’re worried that it might break and can’t live without your custom theme, you can attempt the upgrade in a virtual machine (GNOME Boxes, VirtualBox, etc.) and see how it goes.