3D Graphics are dark

This is a hardware/software issue in the new macOS, I think, and the Metal 3D rendering engine which is now the default.

This comment indicates WRI is aware of the issue.

We can set this option using the "3DRenderingEngine" option to RenderingOptions. This can be done via

SetOptions[$FrontEnd,
  RenderingOptions -> {
    "RenderingEngine" -> <engine>
    }
  ];

or done at the Cell/Notebook level in case WRI patches this for 12.2

Here's what the options do for me

Grid@Transpose@Table[
   {
    o,
    Style[
     Graphics3D[{
       Sphere[]
       }],
     RenderingOptions -> {
       "3DRenderingEngine" -> o
       }
     ]
    },
   {o, {Automatic, "Metal", "OpenGL", "Mesa"}}
   ]

"OpenGL" is faster than "Mesa", but is Unix only. On the other hand, I think Windows doesn't have this issue, so "OpenGL" seems to be the way to go.

I just set

SetOptions[$FrontEnd,
  RenderingOptions -> {
    "RenderingEngine" -> "OpenGL"
    }
  ];

so that I don't need to deal with this again until I update Mathematica

You might try to play with the Lighting option. For example, like this:

Plot3D[Sin[1.75 (x + y)], {x, -5, 5}, {y, -5, 5}, 
 Lighting -> {{"Point", Yellow, {-4, -5, 8}}}]

and try to play with the position of the point light source.

Another possibility is the "Neutral" light source

Plot3D[Sin[1.75 (x + y)], {x, -5, 5}, {y, -5, 5}, 
 Lighting -> "Neutral"]

or a "Directional" one

Plot3D[Sin[1.75 (x + y)], {x, -5, 5}, {y, -5, 5}, 
 Lighting -> {{"Directional", 
    RGBColor[1, 0.5, 0.3], {{5, 5, 7}, {5, 9, 0}}}}]

Have fun!

I see something similar, under macOS, with:

Plot3D[Sin[1.75 (x + y)], {x, -5, 5}, {y, -5, 5}]

I think it's just a matter of the direction of the light source(s).

Rotating the plot manually with the mouse, or changing the ViewPoint will lighten the plot.