style: Add basic support for dark form controls to nsNativeBasicTheme

For that:

 * Tweak the standin system colors to match the non-native theme.

 * Use those system colors for button and field backgrounds.

 * Rename the "should use system colors" bit to "is high contrast",
   which is what it really is (specially now that we use system colors
   also in non-high-contrast).

Border colors and other colors like the <input type=range> and such
might need some extra tweaking perhaps, but this is a decent start and
looks good in https://crisal.io/tmp/form-controls.html afaict (dark mode
toggle needs the color-scheme pref enabled of course).

Differential Revision: https://phabricator.services.mozilla.com/D127533

components/style/values/specified/color.rs

@@ -316,8 +316,12 @@ pub enum SystemColor {
     Captiontext,
     #[parse(aliases = "-moz-field")]
     Field,
+    /// Used for disabled field backgrounds.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozDisabledfield,
     #[parse(aliases = "-moz-fieldtext")]
     Fieldtext,
+
     Graytext,
     Highlight,
     Highlighttext,
@@ -382,6 +386,10 @@ pub enum SystemColor {
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozButtonactiveface,
 
+    /// Used for button background when disabled.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozButtondisabledface,
+
     /// Background color of chrome toolbars in active windows.
     MozMacChromeActive,
     /// Background color of chrome toolbars in inactive windows.