Some textures for the Minecraft Realms screen cannot be changed by resource packs. So far I've seen this only with the 'Minecraft Realms' title and the invitations icon.

Attempting to change these two textures (via resource pack) within gui/title/minecraft.png and gui/widgets.png respectively does not change them ingame.

Attached is a screen capture of the issue with views of both texture files. Red lines indicate the issue & corresponding locations of the textures. Green line shows that the widgets image is indeed from the same resource pack.

I can provide more information if needed, however the resource pack files will need to wait a few days as I'm finishing up to release the next public update.