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In Advanced, various settings can be set and saved as a preset. Beware, you may cause some strange looking effects. Sun colour, sun ambience, fog colour and depth as well as volumetric light colour and density can be set to custom values.
The new layout looks great but more importantly custom skyboxes are here y'all! I'll be playing around with these for a long while :) Thank you u/JagexEdge!
Question edit: The values I alter affect the lighting/color of the ground, but the sky itself remains unchanged. When I alter a value, I can actually see a brief moment of change to the sky, but then the sky reverts to how it used to be, while the ground and objects on ground are changed. Is this intentional? It feels like the sky is rejecting the changes :D
A couple of pics to help illustrate my point. 1st picture is altered dawn, 2nd is default. The changes don't apply to the sky.
I am afraid the skybox is lit independently to environment. It would be best to pick a skybox that fits the appearance you are looking for.
But am glad you like it :)