Better dark mode icon color

[piroor]

Which issue(s) this PR fixes:

N/A

What this PR does / why we need it:

This applies more visible color for icons in the dark mode.

How to verify the fixed issue:

See icons with the dark mode of your platform.

The steps to verify:

1. Change the color scheme of the platform to the dark mode.
2. Open add-ons manager and look at the icon.
3. Try to send a message with external domains.

Expected result:

Icons are rendered with visible color:

On the other hand, the latest release shows icons with hard-to-see color like:

[kenhys]

Confirmed.

LGTM.

diff -u icon1.svg icon1.after.svg
--- icon1.svg   2023-10-19 18:34:56.323045094 +0900
+++ icon1.after.svg     2023-10-19 18:34:31.179027876 +0900
@@ -2,6 +2,19 @@
 <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px"
         y="0px" width="32px" height="32px" viewBox="0 0 32 32" enable-background="new 0 0 32 32" xml:space="preserve">
+<style>
+  :root {
+    color-scheme: light dark;
+  }
+  path {
+    fill: #0C0C0D;
+  }
+  @media (prefers-color-scheme: dark) {
+    path {
+      fill: #F9F9FA;
+    }
+  }
+</style>
 <path d="M24.006,31.998c-0.266,0-0.52-0.105-0.707-0.293l-3-3c-0.384-0.397-0.373-1.03,0.024-1.414c0.388-0.374,1.002-0.374,1.39,0
        l2.157,2.157l6.315-9.023c0.321-0.449,0.945-0.554,1.395-0.233c0.444,0.316,0.553,0.93,0.244,1.38l-7,10
        c-0.168,0.243-0.436,0.399-0.731,0.427C24.064,32,24.035,32,24.006,31.998z"/>
 kenhys@jet  /legacy/thunderbird/flexconfirmmail/Thunderbird.work/webextensions/resources   master 4.1.7 ?  diff -u icon2.svg icon2.after.svg
--- icon2.svg   2023-10-19 18:34:56.323045094 +0900
+++ icon2.after.svg     2023-10-19 18:34:42.059035309 +0900
@@ -2,6 +2,19 @@
 <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
 <svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px"
         y="0px" width="32px" height="32px" viewBox="0 0 32 32" enable-background="new 0 0 32 32" xml:space="preserve">
+<style>
+  :root {
+    color-scheme: light dark;
+  }
+  path {
+    fill: #0C0C0D;
+  }
+  @media (prefers-color-scheme: dark) {
+    path {
+      fill: #F9F9FA;
+    }
+  }
+</style>
 <path d="M20.64,31.998c-0.376,0-0.737-0.15-1.004-0.417l-4.263-4.263c-0.545-0.565-0.531-1.465,0.034-2.01
        c0.551-0.532,1.424-0.532,1.975,0l3.065,3.065l8.975-12.823c0.457-0.638,1.343-0.786,1.981-0.331
        c0.631,0.45,0.785,1.321,0.348,1.961l-9.948,14.209c-0.237,0.346-0.619,0.568-1.039,0.607C20.725,32,20.682,32,20.64,31.998z"/>

[piroor]

Thanks!