The normal theme shipped with KaOS for GTK apps are Breeze. I noticed some problems with the colors when using Firefox and GIMP. The background color of the windows are slightly darker than the window border when using light Midna colors. Also, the tool tips are not readable in GIMP (at least for me).
The easiest way I found to fix this is to copy Breeze theme from /usr/share/themes/Breeze to ~/.themes and rename Breeze to Midna. Then edit ~/.themes/Midna/gtk-2.0/gtkrc and paste the following and save it;
# Theme: Midna-gtk
# Description: Midna theme for GTK+2.0
gtk-color-scheme = "text_color:#31363b"
gtk-color-scheme = "base_color:#fcfcfc"
gtk-color-scheme = "insensitive_base_color:#e5e5e5"
gtk-color-scheme = "fg_color:#31363b"
gtk-color-scheme = "bg_color:#f5f5f5"
gtk-color-scheme = "selected_fg_color:#f5f5f5"
gtk-color-scheme = "selected_bg_color:#3daee9"
gtk-color-scheme = "button_fg_color:#31363b"
gtk-color-scheme = "tooltip_fg_color:#ffffff"
gtk-color-scheme = "tooltip_bg_color:#6b7474"
gtk-color-scheme = "insensitive_fg_color:#aaadaf"
gtk-color-scheme = "insensitive_text_color:#b3b4b6"
gtk-color-scheme = "button_insensitive_fg_color:#aaadaf"
gtk-color-scheme = "button_active:#3daee9"
gtk-color-scheme = "border_color:#bfc1c3"
include "widgets/default"
include "widgets/buttons"
include "widgets/menu"
include "widgets/entry"
include "widgets/notebook"
include "widgets/range"
include "widgets/scrollbar"
include "widgets/toolbar"
include "widgets/progressbar"
include "widgets/misc"
include "widgets/styles"
Go into Settings > Application Appearance > GTK and select the new theme you created (Midna) for GTK2 and GTK3.
This should fix the color problem with GTK applications like GIMP and Firefox.