fix(Datepicker): update 'today' cell background color in dark-v2 theme to increase contrast

[YuanboXue-Amber]

Pull request checklist

• Addresses an existing issue: Fixes React-northstar Datepicker - current day indicator contrast #18077
• Include a change request file using $ yarn change

Description of changes

Datepicker's 'today' cell uses default backgroundFocus as background color. But it does not have enough contrast in dark-v2 theme:

This PR added override in dark-v2 theme, with default background as background color:

based on the suggestions in the original issue

Focus areas to test

(optional)

[size-auditor]

Asset size changes

Size Auditor did not detect a change in bundle size for any component!

Baseline commit: 6ef805ad0bda6dcde322ddb66587a1a9f0878dcd (build)

[fabricteam]

Perf Analysis (@fluentui/react)

No significant results to display.

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	841	803	5000
BaseButton	mount	934	923	5000
Breadcrumb	mount	2638	2624	1000
ButtonNext	mount	463	505	5000
Checkbox	mount	1504	1515	5000
CheckboxBase	mount	1289	1257	5000
ChoiceGroup	mount	4638	4658	5000
ComboBox	mount	959	973	1000
CommandBar	mount	10073	10112	1000
ContextualMenu	mount	6080	6126	1000
DefaultButton	mount	1107	1111	5000
DetailsRow	mount	3663	3730	5000
DetailsRowFast	mount	3677	3654	5000
DetailsRowNoStyles	mount	3552	3443	5000
Dialog	mount	2125	2109	1000
DocumentCardTitle	mount	142	152	1000
Dropdown	mount	3179	3166	5000
FocusTrapZone	mount	1761	1799	5000
FocusZone	mount	1848	1773	5000
IconButton	mount	1700	1713	5000
Label	mount	334	322	5000
Layer	mount	1743	1742	5000
Link	mount	444	446	5000
MakeStyles	mount	1741	1779	50000
MenuButton	mount	1442	1414	5000
MessageBar	mount	2019	1976	5000
Nav	mount	3173	3173	1000
OverflowSet	mount	1029	1016	5000
Panel	mount	2054	2050	1000
Persona	mount	815	821	1000
Pivot	mount	1352	1372	1000
PrimaryButton	mount	1287	1273	5000
Rating	mount	7670	7464	5000
SearchBox	mount	1278	1295	5000
Shimmer	mount	2448	2433	5000
Slider	mount	1919	1950	5000
SpinButton	mount	4859	4811	5000
Spinner	mount	418	402	5000
SplitButton	mount	3066	3084	5000
Stack	mount	473	486	5000
StackWithIntrinsicChildren	mount	1486	1475	5000
StackWithTextChildren	mount	4413	4395	5000
SwatchColorPicker	mount	9925	9939	5000
Tabs	mount	1383	1347	1000
TagPicker	mount	2360	2396	5000
TeachingBubble	mount	11671	11700	5000
Text	mount	403	412	5000
TextField	mount	1328	1338	5000
ThemeProvider	mount	1167	1178	5000
ThemeProvider	virtual-rerender	584	593	5000
ThemeProviderNext	mount	7069	7038	5000
Toggle	mount	800	784	5000
buttonNative	mount	112	117	5000

Perf Analysis (@fluentui/react-northstar)

Perf tests with no regressions

Scenario	Current PR Ticks	Baseline Ticks	Ratio
ButtonMinimalPerf.default	164	153	1.07:1
ReactionMinimalPerf.default	376	352	1.07:1
ListNestedPerf.default	549	520	1.06:1
AvatarMinimalPerf.default	191	183	1.04:1
CardMinimalPerf.default	547	524	1.04:1
ImageMinimalPerf.default	375	361	1.04:1
LayoutMinimalPerf.default	370	356	1.04:1
PortalMinimalPerf.default	175	168	1.04:1
SegmentMinimalPerf.default	345	332	1.04:1
SkeletonMinimalPerf.default	357	344	1.04:1
TableMinimalPerf.default	419	402	1.04:1
AttachmentSlotsPerf.default	1132	1103	1.03:1
ChatWithPopoverPerf.default	361	352	1.03:1
FlexMinimalPerf.default	277	268	1.03:1
HeaderSlotsPerf.default	760	735	1.03:1
MenuMinimalPerf.default	848	826	1.03:1
SliderMinimalPerf.default	1594	1546	1.03:1
TextMinimalPerf.default	351	341	1.03:1
ButtonOverridesMissPerf.default	1673	1642	1.02:1
DropdownManyItemsPerf.default	679	668	1.02:1
GridMinimalPerf.default	334	327	1.02:1
HeaderMinimalPerf.default	354	347	1.02:1
LabelMinimalPerf.default	378	370	1.02:1
LoaderMinimalPerf.default	695	682	1.02:1
PopupMinimalPerf.default	570	557	1.02:1
TableManyItemsPerf.default	1874	1840	1.02:1
ToolbarMinimalPerf.default	923	904	1.02:1
TreeMinimalPerf.default	804	789	1.02:1
DialogMinimalPerf.default	755	748	1.01:1
FormMinimalPerf.default	393	390	1.01:1
InputMinimalPerf.default	1255	1248	1.01:1
ItemLayoutMinimalPerf.default	1249	1237	1.01:1
ProviderMergeThemesPerf.default	1670	1655	1.01:1
ProviderMinimalPerf.default	959	945	1.01:1
RefMinimalPerf.default	243	241	1.01:1
SplitButtonMinimalPerf.default	3692	3648	1.01:1
TooltipMinimalPerf.default	971	960	1.01:1
AttachmentMinimalPerf.default	152	152	1:1
CarouselMinimalPerf.default	457	458	1:1
ChatDuplicateMessagesPerf.default	284	284	1:1
DatepickerMinimalPerf.default	5377	5370	1:1
EmbedMinimalPerf.default	4075	4077	1:1
TextAreaMinimalPerf.default	488	489	1:1
AnimationMinimalPerf.default	403	409	0.99:1
BoxMinimalPerf.default	341	345	0.99:1
ChatMinimalPerf.default	599	604	0.99:1
DropdownMinimalPerf.default	3038	3062	0.99:1
ListCommonPerf.default	615	624	0.99:1
StatusMinimalPerf.default	655	661	0.99:1
CustomToolbarPrototype.default	3739	3790	0.99:1
ButtonSlotsPerf.default	547	557	0.98:1
CheckboxMinimalPerf.default	2707	2755	0.98:1
ListMinimalPerf.default	493	502	0.98:1
MenuButtonMinimalPerf.default	1540	1565	0.98:1
IconMinimalPerf.default	597	611	0.98:1
AccordionMinimalPerf.default	146	151	0.97:1
RadioGroupMinimalPerf.default	431	444	0.97:1
AlertMinimalPerf.default	266	277	0.96:1
RosterPerf.default	1126	1169	0.96:1
TreeWith60ListItems.default	161	168	0.96:1
DividerMinimalPerf.default	345	363	0.95:1
ListWith60ListItems.default	598	645	0.93:1
VideoMinimalPerf.default	612	665	0.92:1