can we change color of the "Accept this task ..." text and "Accept" button in a task? without having to change the theme of brand.

I think for that you would have to look into changing the Hex values for the theme accent colors. That would change the color of the button and possibly "Accept this task ..." without changing the colors of the page background. I don't know of a way to change the colors of a specific button.

Theme will changes colour of all buttons. Need only one button to change. I think we don't have that option.

It is not possible to change these colors and format without changing the branding scheme. I would ABSOLUTELY NOT suggest this, but you theoretically could change the tasks to a database queue approach instead of Appian OOTB group task assignment, and then create a card layout to mimic the task acceptance. You can then format the card whichever way you prefer.

Database queues are often a good design pattern for application development, but it would take a significant redesign, and it is definitely not worth it just for changing the color scheme.