Hi @Sushrut Paranjape ,
As far as I know, there is no such way to apply a particular theme on page load. Applying theme is on site level.
We could only get the current theme properties and use it in spfx customizations, you could refer to this article for more: https://video2.skills-academy.com/en-us/sharepoint/dev/spfx/use-theme-colors-in-your-customizations
Besides, the spfx Theme Manager would be helpful to you: https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/js-theme-manager
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