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

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.

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.