Appearance change Gimp 2.10 (theme) on 18.04

If the installation was done via flatpak you can change the look


The theme I edited is Dark and we find it by following the path:

Inside the Dark folder has a gtkrc file.
Opening (as root) the file has the colors that will be displayed.
color["clr1"] = "#DDDDDD"
color["clr2"] = "#FFFFFF"
color["clr3"] = "#C1C1C1"
color["clr4"] = "#383838"
color["clr5"] = "#b23a0d"
color["clr6"] = "#828282"
color["clr7"] = "#3B3B3B"
color["clr8"] = "#383838"
color["clr9"] = "#454545"
color["clrA"] = "#303030"
I just changed the clr5 from b23a0d to 555555.
By putting the value eg C1C1C1 on the color selector we see the color!