Fix Caps-lock-is-on color in light theme

When a Polkit dialog is open to ask for the password, if the
user activates the Caps Lock, a message appears under the text
entry notifying that it is active, to warn the user.

In dark theme, this message is painted in yellow, which is fine
because it gives a good contrast against the background; but in
litht theme, a slightly darker yellow is used, which has poor
contrast. This goes against the accessibility (A11Y) requirements
of having, at least, a 4.5/1 contrast ratio in text.

This patch fixes this by setting the WARNING text color to black.

Author: sergio-costas

gnome-shell/src/gnome-shell-sass/_default-colors.scss

@@ -17,7 +17,7 @@ $success_bg_color: if($variant == 'light', $green_4, $green_5);
 $success_fg_color: $light_1;
 $success_color: $success_bg_color;
 
-$warning_bg_color: if($variant == 'light', $yellow_5, #cd9309); // uses darker off-palette yellow
+$warning_bg_color: if($variant == 'light', black, #cd9309); // uses darker off-palette yellow
 $warning_fg_color: transparentize(black, .2);
 $warning_color: $warning_bg_color;
 

gnome-shell/src/gnome-shell-sass/_yaru-default-colors.scss

@@ -6,4 +6,4 @@ $_base_color_light: $light_1; // Yaru: use our colors
 $yaru_borders_color_dark: lighten(desaturate(lighten($jet, 4%), 100%), 14%);
 
 // Overrides default
-$warning_bg_color: if($variant =='light', darken($yellow, 1%), $yellow);
+$warning_bg_color: if($variant =='light', black, $yellow);

gnome-shell/upstream/theme/gnome-shell-sass/_default-colors.scss

@@ -17,7 +17,7 @@ $success_bg_color: if($variant == 'light', $green_4, $green_5);
 $success_fg_color: $light_1;
 $success_color: $success_bg_color;
 
-$warning_bg_color: if($variant == 'light', $yellow_5, #cd9309); // uses darker off-palette yellow
+$warning_bg_color: if($variant == 'light', black, #cd9309); // uses darker off-palette yellow
 $warning_fg_color: transparentize(black, .2);
 $warning_color: $warning_bg_color;