(core) Enable the 'none' option in ColorSelect for cell and header text styles

Test Plan: Added a test case

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4120

app/client/ui2018/ColorSelect.ts

@@ -324,7 +324,7 @@ class PickerComponent extends Disposable {
           )
         ),
         cssEmptyBox(
-          cssEmptyBox.cls('-selected', (use) => !use(this._colorCss)),
+          cssEmptyBox.cls('-selected', (use) => !use(this._colorHex)),
           dom.on('click', () => this._setValue(undefined)),
           dom.hide(!this._options.allowsNone),
           cssNoneIcon('Empty'),

app/client/widgets/CellStyle.ts

@@ -54,6 +54,7 @@ export class CellStyle extends Disposable {
                   textColor: new ColorOption({
                     color: headerTextColor,
                     defaultColor: theme.tableHeaderFg.toString(),
+                    allowsNone: true,
                     noneText: 'default',
                   }),
                   fillColor: new ColorOption({
@@ -109,6 +110,7 @@ export class CellStyle extends Disposable {
               textColor: new ColorOption({
                 color: textColor,
                 defaultColor: this._defaultTextColor,
+                allowsNone: true,
                 noneText: 'default',
               }),
               fillColor: new ColorOption({