Add a column to your links data.frame that will specify the group for each link (in your case, they will all be in the same group). Then add a custom color scale command that will specify "blue" for that group. Then tell the sankeyNetwork() function the name of the column in your links data.frame that specifies the link group and pass it you custom colorScale.

library("networkD3")

data <- read.csv(header = TRUE, text = "
date,Data Center,Customer,companyID,source,target,value 
6/1/2021,dcA,customer1,companyID1,open_list_view_1,open_card_2,1 
6/1/2021,dcA,customer1,companyID1,open_card_2,edit_card_3,1
6/1/2021,dcA,customer1,companyID1,edit_card_3,save_card_4,1 
6/1/2021,dcA,customer1,companyID1,save_card_4,back_to_card_list_5,2 
6/1/2021,dcA,customer1,companyID1,back_to_card_list_5,show_more_6,1
6/1/2021,dcA,customer1,companyID1,show_more_6,view_introduction_7,1
6/1/2021,dcA,customer1,companyID1,view_introduction_7,scroll_down_8,2
6/2/2021,dcA,customer2,companyID2,open_list_view_1,open_card_2,3
6/2/2021,dcA,customer2,companyID2,open_card_2,edit_card_3,1
6/2/2021,dcA,customer2,companyID2,edit_card_3,save_card_4,4
6/2/2021,dcA,customer2,companyID2,save_card_4,back_to_card_list_5,2 
6/2/2021,dcA,customer2,companyID2,back_to_card_list_5,show_more_6,1
6/2/2021,dcA,customer2,companyID2,show_more_6,view_introduction_7,1
6/2/2021,dcA,customer2,companyID2,view_introduction_7,scroll_down_8,5
")

node_names <- unique(c(as.character(data$source), as.character(data$target)))
nodes <- data.frame(name = node_names)
links <- data.frame(source = match(data$source, node_names) - 1,
                    target = match(data$target, node_names) - 1,
                    value = data$value)

links$linkgroup <- "linkgrp"

colourScale <- 
  'd3.scaleOrdinal()
     .domain(["linkgrp"])
     .range(["blue"].concat(d3.schemeCategory20))'

sankeyNetwork(Links = links, Nodes = nodes, Source = "source",
              Target = "target", Value = "value", NodeID = "name",
              sinksRight = FALSE, 
              LinkGroup = "linkgroup", 
              colourScale = colourScale)