Update picker headers styling in Darcula themes (#11620)

* Apply styling to picker headers in Darcula themes

* Add background to active picker column in Darcula.

runtime/themes/darcula.toml

@@ -28,6 +28,8 @@
 "ui.virtual.jump-label" = { fg = "lightblue", modifiers = ["italic", "bold"] }
 "ui.bufferline" = { fg = "grey04", bg = "grey00" }
 "ui.bufferline.active" = { fg = "grey07", bg = "grey02" }
+"ui.picker.header.column" = { fg = "grey05", modifiers = ["italic", "bold"] }
+"ui.picker.header.column.active" = { fg = "grey05", bg = "grey03", modifiers = ["italic", "bold"] }
 
 "operator" = "grey05"
 "variable" = "white"