Here's how I do it programatically inside the slider script, based on the other answer.
I don't know if there's a better way to do it.
var mainstyle = StyleBoxFlat.new()
var grstyle = StyleBoxFlat.new()
#Set Background Color
mainstyle.bg_color = Color(0, 0, 0) #Black
#Set Grabber Area Background Color
grstyle.bg_color = Color(1, 1, 1) #White
#Make both areas expand from the center and fill the rect_size
grstyle.expand_margin_bottom = rect_size.y / 2
grstyle.expand_margin_top = rect_size.y / 2
mainstyle.expand_margin_bottom = rect_size.y / 2
mainstyle.expand_margin_top = rect_size.y / 2
#Fake a fancy grabber using Grabber Area's right border (optional)
grstyle.border_color = Color(0.5, 0, 0) #Dark Red
grstyle.border_width_right = 10
Note: This is for Horizontal Slider, In vertical i guess you only have to change
Note2: The faked fancy grabber is currently the Grabber Area Texture border, so if you plan to add borders to the slider you won't be able to set a different color (maybe there are workarounds like padding a bit and using the Main border... whatever, that's another thing haha)
I'm currently struggling to get rid of the grabber entirely from script (currently setting theme to a theme I saved with all the grabbers textures set to a new image texture) and make a new one (maybe via
draw_rect()?) without messing up the borders.