Changing the border width of a plot

I have a script to create an easy plot, but I would like to change the border width because it is a little bit transparent. How can I do this?
With images is clearer the my problem

df_stats
import matplotlib.pyplot as plt
import numpy as np
plt.figure()
ax = plt.axes()

# Increase the plot size and font size.
plt.rcParams["figure.figsize"] = (300,200)
plt.title("Training & Validation Loss",fontsize=500)
plt.xlabel("Epoch", fontsize=500)
plt.ylabel("Loss", fontsize=500)
plt.xticks(list(np.arange(0, 30, 5)))
plt.xticks(fontsize=500)
plt.yticks(fontsize=500)
plt.plot(df_stats['Training Loss'], color='b', marker='.', linestyle='solid', mec='b', markersize=200, markerfacecolor='white', label="Training", linewidth=80)
plt.plot(df_stats['Valid. Loss'], color='g', marker='.', linestyle='solid',  mec='b', markersize=200, markerfacecolor='white',label="Validation", linewidth=80)
plt.grid(True, color='black', linestyle='dashed', linewidth=10, axis='x')
plt.grid(True, color='black', linestyle='dashed', linewidth=10, axis='y')  
plt.legend(fontsize=500, edgecolor='black')

>Solution :

I changed the size of your plot because it was way too huge for me to work with, just FYI.

What you can do is access the spines on your figure by using ax.spines. To do this, first you have to place fig, ax = plt.subplots() at the top of your plot code (so that we can use ax). Then, we can change the line width of the spines to whatever we want with set_linewidth(). Unfortunately, I don’t know a good way to include all the spine locations in one line of code, so I had to put it in a for loop:

for location in ['left', 'right', 'top', 'bottom']:
    ax.spines[location].set_linewidth(10)

So, altogether, it would look like this:

plt.rcParams["figure.figsize"] = (30,20)

fig, ax = plt.subplots()
plt.title("Training & Validation Loss",fontsize=50)
plt.xlabel("Epoch", fontsize=50)
plt.ylabel("Loss", fontsize=50)
plt.xticks(list(np.arange(0, 30, 5)), alpha=1)
plt.xticks(fontsize=50)
plt.yticks(fontsize=50)
plt.plot(np.arange(0, 30, 5), color='b', marker='.', linestyle='solid', mec='b', markersize=20, markerfacecolor='white', label="Training", linewidth=8)
plt.plot(np.arange(0, 30, 5), color='g', marker='.', linestyle='solid',  mec='b', markersize=20, markerfacecolor='white',label="Validation", linewidth=8)
plt.grid(True, color='black', linestyle='dashed', linewidth=1, axis='x')
plt.grid(True, color='black', linestyle='dashed', linewidth=1, axis='y')  
plt.legend(fontsize=20, edgecolor='black')
for location in ['left', 'right', 'top', 'bottom']:
    ax.spines[location].set_linewidth(10)