If you've been visiting GameMaker Community lately, you might've noticed that forum software update has switched the light theme to new default IPB blue theme.
That is going to be fixed soon (hopefully!), but until then, there's a small solution if you're not too keen of mixed standard blue-green-pink(?!) palette.
First, ensure that you have a browser plugin to add support for userstyle. Seems that a commonly used one is Stylish, but there are also other options.
Also ensure that forum is set to default (blue) theme.
Then, create a new style in said plugin.
"Applies to domain" field would to be set to "gmc.yoyogames.com", obviously.
CSS contents can be downloaded here:
After clicking "Save" and refreshing page, theme will appear green-coloured, more or less mimicking the previously used green theme (except maybe slightly softer, since it's a quick CSS recolour of new standard theme).
Optionally, you can also download a ZIP archive with related files, so you can easily substitute links in CSS and host images wherever loads fastest for you.
Should be good.