Switch default theme to "Next" from "V7"

[KrooshalUX]

• In version release 2.10, users should see "Next" theme as provided by OUI theme system as default theme
• Users who have previous installations of OpenSearch Dashboards should have their previously chosen Dark or Light mode theme preferences retained

[mustafaazad03]

Hi,
I understand this issue and I would like to solve this issue.

[wbeckler]

@mustafaazad03, what approach were you thinking?

[joshuarrrr]

@mustafaazad03 Thanks for volunteering! We'll need this on a relatively short deadline. Do you think you'll be able to complete it this week? I'll go ahead and assign it for now, but we may have someone else grab it if the timeline doesn't work for you.

[mustafaazad03]

@mustafaazad03, what approach were you thinking?

I am thinking that what changes were made earlier so that this issue happened and accordingly I make some changes so that this issue will resolve without affecting any new update.

[joshuarrrr]

I went ahead and created the PR.