Fix form border color and remove unnecessary borderDisabledColor

- transparency is way not low enough right now, it needs to be `0.1` in order to not sit on top of the underline background linear gradient

- as a result we need to darken the heck out of the border color - still not a 1:1 migration as the old color was semi-ludicrous, but definitely close enough

+ add `border: none` CSS to make round out the shadow-related logic and replace static `1px` with a token

packages/eui/src/components/form/form.styles.test.tsx

@@ -30,8 +30,7 @@ describe('euiFormVariables', () => {
         "backgroundColor": "#f9fbfd",
         "backgroundDisabledColor": "#eef1f7",
         "backgroundReadOnlyColor": "#FFF",
-        "borderColor": "rgba(211,218,230,0.9)",
-        "borderDisabledColor": "rgba(211,218,230,0.9)",
+        "borderColor": "rgba(32,38,47,0.1)",
         "controlBorderRadius": "6px",
         "controlBoxShadow": "0 0 transparent",
         "controlCompressedBorderRadius": "4px",

packages/eui/src/components/form/form.styles.ts

@@ -44,8 +44,7 @@ export const euiFormVariables = (euiThemeContext: UseEuiTheme) => {
     backgroundColor: backgroundColor,
     backgroundDisabledColor: darken(euiTheme.colors.lightestShade, 0.05),
     backgroundReadOnlyColor: euiTheme.colors.emptyShade,
-    borderColor: transparentize(euiTheme.border.color, 0.9),
-    borderDisabledColor: transparentize(euiTheme.border.color, 0.9),
+    borderColor: transparentize(darken(euiTheme.border.color, 4), 0.1),
     controlDisabledColor: euiTheme.colors.mediumShade,
     controlBoxShadow: '0 0 transparent',
     controlPlaceholderText: makeHighContrastColor(euiTheme.colors.subduedText)(
@@ -181,7 +180,9 @@ export const euiFormControlDefaultShadow = (euiThemeContext: UseEuiTheme) => {
   const form = euiFormVariables(euiThemeContext);
 
   return `
-    box-shadow: inset 0 0 0 1px ${form.borderColor};
+    /* We use inset box-shadow instead of border to skip extra hight calculations */
+    border: none;
+    box-shadow: inset 0 0 0 ${euiTheme.border.width.thin} ${form.borderColor};
     background-color: ${form.backgroundColor};
 
     background-repeat: no-repeat;