If you're alright with imprecise solutions, setting form=True on the 
dbc.Row object will tighten the gutters up.

# For more fine-grained control, set no_gutters=True as was recommended in 
# the other answer, and on the dbc.Col objects add 
style={'margin-right': '5px', 'margin-left': '5px'}. 
# You can adjust those as appropriate. For end columns, you may want to 
# remove the margin on the end.