Separate styling options for major/minor grid line in rcParams

Using rcParams["grid.major.*"] or rcParams["grid.minor.*"] will overwrite the value in rcParams["grid.*"] for the major and minor gridlines, respectively.

import matplotlib as mpl
import matplotlib.pyplot as plt


# Set visibility for major and minor gridlines
mpl.rcParams["axes.grid"] = True
mpl.rcParams["ytick.minor.visible"] = True
mpl.rcParams["xtick.minor.visible"] = True
mpl.rcParams["axes.grid.which"] = "both"

# Using old values to set both major and minor properties
mpl.rcParams["grid.color"] = "red"
mpl.rcParams["grid.linewidth"] = 1

# Overwrite some values for major and minor separately
mpl.rcParams["grid.major.color"] = "black"
mpl.rcParams["grid.major.linewidth"] = 2
mpl.rcParams["grid.minor.linestyle"] = ":"
mpl.rcParams["grid.minor.alpha"] = 0.6

plt.plot([0, 1], [0, 1])

plt.show()

(Source code, 2x.png, png)