preferences: Increase contrast of setting text and nav items.

The text in the settings panel was unnecessarily fade, making it hard to
read. Increasing the contrast makes it more readable. The setting text
color is the same as the heading of the settings. This commit also
changes the color of the nav items to be the same as `|` bar preceding
it.
https://chat.zulip.org/#narrow/channel/101-design/topic/zulip-desktop.20preferences.20contrast

Author: shubham-padia

app/renderer/css/preference.css

@@ -11,7 +11,7 @@
   background: rgb(239 239 239 / 100%);
   letter-spacing: -0.08px;
   line-height: 18px;
-  color: rgb(139 142 143 / 100%);
+  color: rgb(34 44 49 / 100%);
 
   /* Copied from https://github.com/yairEO/tagify/blob/v4.17.7/src/tagify.scss#L4-L8 */
   --tagify-dd-color-primary: rgb(53 149 246);
@@ -101,7 +101,7 @@ td:nth-child(odd) {
 
 .nav {
   padding: 7px 0;
-  color: rgb(153 153 153 / 100%);
+  color: rgb(70 78 90 / 100%);
   cursor: pointer;
 }