Firefox not affected by gtk theme

Solution 1:

You can use Gnome Tweak Tool to use the dark theme for all the applications:

BUT

Firefox uses GTK2. Gnome Tweak Tool only edits the GTK3 settings, hence we need to edit the /usr/share/themes/Adwaita/gtk-2.0/gtkrc file.

1. Initially we backup our gtkrc file

   sudo cp /usr/share/themes/Adwaita/gtk-2.0/gtkrc /usr/share/themes/Adwaita/gtk-2.0/gtkrc.bak
   

2. Next copy the following and replace your existing gtkrc file:

   #Color scheme originally contributed by Franco Gotusso as Clearlooks-DarkOrange 
   # Based on Clearlooks-DarkCoffee by Chibi and edited by bvc as Clearlooks-DarkCoffee2
   
   style "clearlooks-default"
   {
     GtkMenuItem::selected_shadow_type = none
     GtkWidget::interior_focus = 1
     GtkButton::default_border = { 3, 3, 3, 3 }
     GtkButton::default_outside_border = { 3, 3, 3, 3 }
     GtkRange::trough_border = 2
   
     GtkWidget::focus_padding = 1
   
     GtkPaned::handle_size = 6
   
     GtkRange::slider_width = 15
     GtkRange::stepper_size = 15
     GtkScrollbar::min_slider_length = 30
     GtkCheckButton::indicator_size = 12
     GtkMenuBar::internal-padding = 0
   
     GtkTreeView::expander_size = 14
     GtkExpander::expander_size = 16
   
     GtkTreeView::odd_row_color = "#404040"
   
     GtkWidget::cursor_color           = "#72706E"
     GtkWidget::secondary_cursor_color = "#72706E"
   
     xthickness = 1
     ythickness = 1
   
     fg[NORMAL]        = "#d2cfcc" #Main window text
     fg[PRELIGHT]      = "#E9E9E9" #Highlighted widget text
     fg[ACTIVE]        = "#ADA59D" #Inactive widget text
     fg[SELECTED]      = "#E9E9E9"
     fg[INSENSITIVE]   = "#A18989"
   
     bg[NORMAL]        = "#393f3f" #Backround
     bg[PRELIGHT]      = "#3465A4" #Highlight Widget
     bg[ACTIVE]        = "#2E3232" #Selected Widget
     bg[SELECTED]      = "#3465A4" #The box words are usually in
     bg[INSENSITIVE]   = "#282B2B" #Not active buttons
     base[NORMAL]      = "#4F524F" #Text area widgets
     base[PRELIGHT]    = "#313131" #Check and radio button background
     base[ACTIVE]      = "#313636" #Unfocused Select
     base[SELECTED]    = "#2F4C6D" #Selected Text area item, and that bar over the tabs.
     base[INSENSITIVE] = "#f5f2ee" #
   
     text[NORMAL]      = "#EEEEEC" #Text area widget text.
     text[PRELIGHT]    = "#E9E9E9" #
     text[ACTIVE]      = "#ADA59D" #
     text[SELECTED]    = "#E9E9E9" #Selected Text area widget text.
     text[INSENSITIVE] = "#757575" #
   
   engine "clearlooks"
      {
       #    sunkenmenubar      = 1       # 0 = disable, 1 = enable
       #    menuitemstyle      = 1       # 0 = flat, 1 = 3d-ish (button)
       #    listviewitemstyle  = 1       # 0 = flat, 1 = 3d-ish (gradient)
       #    progressbarstyle   = 0       # 0 = candy bar, 1 = flat
     }
   }
   
   style "clearlooks-wide" = "clearlooks-default"
   {
     xthickness = 2
     ythickness = 2
     bg[NORMAL]   = "#414141"
     bg[PRELIGHT] = "#494949"
   }
   
   style "clearlooks-button" = "clearlooks-wide"
   {
     bg[NORMAL]   = "#414141"
     bg[PRELIGHT] = "#494949"
   }
   
   style "clearlooks-notebook" = "clearlooks-wide"
   {
     bg[NORMAL] = "#404040"
     bg[ACTIVE] = "#303030"
   }
   
   style "clearlooks-tasklist" = "clearlooks-default"
   {
     xthickness = 5
     ythickness = 3
   }
   
   style "clearlooks-menu" = "clearlooks-default"
   {
     xthickness = 2
     ythickness = 1
     bg[NORMAL] = "#404040"
   }
   
   style "clearlooks-menu-item" = "clearlooks-default"
   {
     xthickness = 2
     ythickness = 3
     fg[PRELIGHT]   = "#E9E9E9"
     text[PRELIGHT] = "#E9E9E9"
     base[PRELIGHT] = "#3465A4"
     base[SELECTED] = "#3465A4"
   }
   
   style "clearlooks-menu-itembar" = "clearlooks-default"
   {
     xthickness = 3
     ythickness = 3
   }
   
   style "clearlooks-tree" = "clearlooks-default"
   {
     xthickness = 2
     ythickness = 2
   }
   
   style "clearlooks-frame-title" = "clearlooks-default"
   {
     fg[NORMAL] = "#e2dfdc"
   }
   
   style "clearlooks-panel" = "clearlooks-default"
   {
     xthickness = 3
     ythickness = 3
   }
   
   style "clearlooks-tooltips" = "clearlooks-default"
   {
     xthickness = 4
     ythickness = 4
     bg[NORMAL] = "#3465A4"
   }
   
   style "clearlooks-progressbar"  = "clearlooks-default"
   {
     xthickness = 1
     ythickness = 1
      fg[PRELIGHT]         = "#e2dfdc"
      bg[NORMAL]           = "#2d1d11"
   }
   
   style "clearlooks-combo" = "clearlooks-default"
   {
     xthickness = 1
     ythickness = 2
     fg[NORMAL]            = "#141414"
     fg[PRELIGHT]          = "#ed7013"
     fg[ACTIVE]            = "#e2dfdc"
     bg[NORMAL]            = "#414141"
     bg[PRELIGHT]          = "#494949"
   }
   
   style "clearlooks-scrollbar" = "clearlooks-default"
   {
     fg[NORMAL]            = "#141414"
     fg[PRELIGHT]          = "#e2dfdc"
     fg[ACTIVE]            = "#e2dfdc"
     bg[NORMAL]            = "#414141"
     bg[PRELIGHT]          = "#494949"
   }
   
   style "clearlooks-spin" = "clearlooks-wide"
   {
     fg[NORMAL]            = "#141414"
     fg[PRELIGHT]          = "#e2dfdc"
     fg[ACTIVE]            = "#e2dfdc"
     bg[PRELIGHT]          = "#414141"
     bg[ACTIVE]            = "#494949"
   }
   
   class "GtkWidget" style "clearlooks-default"
   class "GtkRange" style "clearlooks-wide"
   class "GtkFrame" style "clearlooks-wide"
   class "GtkSpinButton" style "clearlooks-spin"
   class "GtkStatusbar" style "clearlooks-wide"
   class "GtkMenu" style "clearlooks-menu"
   class "GtkMenuItem" style "clearlooks-menu-item"
   widget_class "*MenuItem.*" style "clearlooks-menu-item"
   class "GtkEntry" style "clearlooks-wide"
   widget_class "*.tooltips.*.GtkToggleButton" style "clearlooks-tasklist"
   widget_class "*.GtkTreeView.GtkButton" style "clearlooks-tree"
   widget_class "*.GtkCTree.GtkButton" style "clearlooks-tree"
   widget_class "*.GtkList.GtkButton" style "clearlooks-tree"
   widget_class "*.GtkCList.GtkButton" style "clearlooks-tree"
   widget_class "*.GtkFrame.GtkLabel" style "clearlooks-frame-title"
   widget_class "BasePWidget.GtkEventBox.GtkTable.GtkFrame" style "clearlooks-panel"
   widget "gtk-tooltips" style "clearlooks-tooltips"
   class "GtkNotebook" style "clearlooks-notebook"
   class "GtkProgressBar" style "clearlooks-progressbar"
   widget_class "*.GtkComboBox.GtkButton" style "clearlooks-combo"
   widget_class "*.GtkCombo.GtkButton" style "clearlooks-combo"
   class "GtkButton" style "clearlooks-button"
   class "GtkScrollbar" style "clearlooks-scrollbar"
   

Solution 2:

On newer Firefox versions (67) GTK3 dark theme support is still rubbish. (Form widgets often render with wrong colors.) Here's a band-aid fix:

Goto: about:config 
Add String entry: widget.content.gtk-theme-override
Set value to: Adwaita:light
Goto Customizations: choose Dark theme