The color and size of the vignette effect is adjustable.
shader_type canvas_item;
uniform vec4 color: hint_color;
uniform float multiplier = 0.2;
uniform float softness = 3.0;
void fragment() {
float value = distance(UV, vec2(0.5));
COLOR = vec4(color.rgb, smoothstep(multiplier, softness, value));
}