Replacing them at the action bar does not make any sense as the bar will be updated after each new skill placed there, sometimes if skills are used and if you change the bars from weapon set 1 to 2 and backwards.
So a redirect might be the best option here.
For other textures where a replacement makes sense:
-Download the addon Zgoo
-Use it ingame, place mouse over a control where you want to find the texture, write
into the chat and press return key
-> As alternative go to the keyboard key settings and specify a keybind for "Zgoo mouse" and use this keybind then ("Numpad 0" is a good one imo)
-Zgoo will open and show you the control below your mosue
The name of the current control will be written at the entry ":GetName()", e.g. ZO_PlayerInventoryMenubarButton1
-Find the texture of this control by opening the :GetChildren() function and see the controls that are children of the current control.
-Select the one named "Image" or "Texture" or simlar, where the type CT_TEXTURE is writtne behind in light blue color.
The selected texture name is the red tetx in front of the CT_TEXTURE, e.g.
ZO_PlayerInventoryMenubarButton1Image
-Press the "+" in front of it to see the controls info of that texture.
-At the function entry :GetTextureFileName() you'll see the .dds file of that texture control
Via ZO_PlayerInventoryMenubarButton1Image:SetTexture(pathToDDSFileName) you're able to set a new dds file to it.