Hi! Anyway to fix this bright green toxic head color to old good dark green color, as it was before 3.4.1 Ulduar patch? Just like on any screenshots (https://www.curseforge.com/wow/addons/skillet-classic/screenshots). Thank you!

You can thank Blizzard for removing SetGradientAlpha from the UI. There were enough other issues that I just removed them rather than research a replacement. Since you have taken the trouble to report it 😀, I have done the research and will return the old look in the next release.

In the mean time, you can pick a color you like by changing line 255 in ...\Addons\Skillet-Classic\UI\MainFrame.lua:
local r,g,b = 0, 0.7, 0; -- dark green

If you are really ambitious, the new "old" look is

	titlebar2:SetHeight(13)
--	titlebar:SetGradientAlpha("VERTICAL", r*0.6,g*0.6,b*0.6,1,r,g,b,1)
	titlebar:SetGradient("VERTICAL", CreateColor(r*0.6,g*0.6,b*0.6,1), CreateColor(r,g,b,1))
	titlebar:SetColorTexture(r,g,b,1)
--	titlebar2:SetGradientAlpha("VERTICAL",r*0.9,g*0.9,b*0.9,1,r*0.6,g*0.6,b*0.6,1)
	titlebar2:SetGradient("VERTICAL", CreateColor(r*0.9,g*0.9,b*0.9,1), CreateColor(r*0.6,g*0.6,b*0.6,1))
	titlebar2:SetColorTexture(r,g,b,1)

Fixed in Skillet-Classic-1.62.