Theme fixes (#214)

* Dark Theme Visual Improvements

-Fixed Container Constants
-Fixed grabber slider
-Added new Grabber icon and split container icon
-Removed button Borders

* Fixed grid containers on all themes and some visual changes on grey and dark theme

* Light and Gold themes fixes

* Update Gold Theme.tres

* Update Dark Theme.tres

Co-authored-by: Erevoid <Erevoid@users.noreply.github.com>
Co-authored-by: Manolis Papadeas <35376950+OverloadedOrama@users.noreply.github.com>

Assets/Graphics/Light Themes/Misc/Vsplit.png.import

@@ -0,0 +1,34 @@
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