How to customise wp-activate / wp-register pages

Hi Guys,

First i'd like to say a big thank you in advance to any help or support you can lend to help me regarding this.

I am trying to find a solution on how to customise the wp-activate.php / wp-register.php pages.

Currently both pages work, however when viewing the pages from a browser on a computer the top menu covers most of the text which is on the pages (I have included a couple of screenshots).

As my theme is responsive, if I visit the pages on a mobile device, this issue doesn't happen, only on a larger screen.

Is there anyway to either hide the menu on these pages, or to push the text down (even using white space) to make the text visible and located directly underneath the menu?

Again, I thank you for your help.