That is the default theme.
I am not sure how it works on the ipad app but in that theme, at bottom left of every page on the forums, there is a link that says
default style.
This is the theme chooser - If you click this it shows a choice of 3
Default style
PC Gamer
PC Gamer Dark
Choosing either of the last 2 will bring the normal branding.
Note: If you on the PC Gamer themes and want to swap, the chooser is on bottom right of all pages. Just be aware the theme chooser on default theme is on left at bottom, not right
Note 2: the label of the theme changer will always show your current themes name.
SHaines and I had fun figuring out how to get back when I showed him the default once before.