This tip was born through some necessity - the new desktop site is very bright indeed with a lot of white space and I find that if I use it full screen it can cause eye strain so I looked for a solution or at least a workaround.
I found that there are two extensions available for Chrome Web Browser for Windows which offer a possible solution by changing web page colours and brightness automatically.
The first Chrome Extension is called ToggleDocumentColors - I have not yet tried them but looks like they do similar things to the above Chrome Extensions.
The Chrome Extensions each perform similar functions - changing web colours to different colour schemes. Each extension allows some customisation and can be set to work (or NOT work) only on specific web sites via whitelist / blacklist logic.
I know that some people struggle with viewing specific colour schemes and this may be a way to deal with that issue. Other sites may have similar issues and of course this advice is not specific to giffgaff's forum pages only.
The main caveat is that sometimes functionality or images can get lost a bit (e.g. a text entry box may be coloured the same as the surrounding background), depending on the scheme and settings used. To counter that it is easy to switch the filters on or off from the browser toolbar.
Also you can even install both extensions in Chrome and switch them on and off independently to experiment.
Here are two screen shots - these are just example colour schemes, there are many more to choose from.