Discrete
If you wish to specify a default, discrete colour scale such as that produced by scale_colour_brewer() use the ggplot2.discrete.colour option. Similarly, use the option ggplot2.discrete.fill for a discrete fill scale.
Default Discrete Scales
library(ggplot2)
ggplot(mtcars, aes(hp, mpg, color = factor(cyl), fill = factor(cyl))) + geom_point()

Created on 2021-07-01 by the reprex package (v1.0.0)
Custom Discrete Scales
library(ggplot2)
scale_colour_brewer_d <- function(...) {
scale_colour_brewer(palette = "Dark2", ...)
}
scale_fill_brewer_d <- function(...) {
scale_fill_brewer(palette = "Dark2", ...)
}
options(
ggplot2.discrete.colour = scale_colour_brewer_d,
ggplot2.discrete.fill = scale_fill_brewer_d
)
ggplot(mtcars, aes(hp, mpg, color = factor(cyl), fill = factor(cyl))) + geom_point()

Created on 2021-07-01 by the reprex package (v1.0.0)
Continuous
If you wish to specify a default, continuous color scale, you will need to use scale_colour_distiller() instead of scale_colour_brewer(). Similarly, use scale_fill_distiller() as opposed to scale_fill_brewer() for a continuous fill scale. You will also use the options ggplot2.continuous.colour and ggplot2.continuous.fill respectively.
Default Continuous Scales
library(ggplot2)
ggplot(mtcars, aes(hp, mpg, color = cyl, fill = cyl)) + geom_point()

Created on 2021-07-01 by the reprex package (v1.0.0)
Custom Continuous Scales
library(ggplot2)
scale_colour_brewer_c <- function(...) {
scale_colour_distiller(palette = "Dark2", ...)
}
scale_fill_brewer_c <- function(...) {
scale_fill_distiller(palette = "Dark2")
}
options(
ggplot2.continuous.colour = scale_colour_brewer_c,
ggplot2.continuous.fill = scale_fill_brewer_c
)
ggplot(mtcars, aes(hp, mpg, color = cyl, fill = cyl)) + geom_point()

Created on 2021-07-01 by the reprex package (v1.0.0)