You should take a look on this, some features you're using are deprecated:
[1/13] Compiling Vala source ../src/Widgets/WebItem.vala .....ttings.vala ../src/MainWindow.vala ../src/Application.vala.
../src/Widgets/Browser.vala:75.36-75.62: warning: Gtk.CssProvider.get_default has been deprecated since 3.24
../src/Windows/WebApp.vala:43.13-43.28: warning: Gtk.Window.set_wmclass has been deprecated since 3.22
../src/Objects/DesktopFile.vala:76.9-76.30: warning: Type `Gdk.RGBA?' can not be used for a GLib.Object property
public Gdk.RGBA? color {
^^^^^^^^^^^^^^^^^^^^^^
Compilation succeeded - 3 warning(s)
You should take a look on this, some features you're using are deprecated: