FEAT: add user-defined bar style

[txtxj]

Is your feature request related to a problem? Please describe.

The default style of the bar is not displayed properly on some devices

I know there are two other styles offered, but I prefer the first default style (if it doesn't display wrongly).

Describe the solution you'd like

I think it is necessary to allow user-defined styles. An interface could be provided for users to enter custom black and white blocks

[caupolicanre]

Hi! i'm trying to improve the symbol handling. I've submited this issue #490 with the new enviroment variable and the logic improvement, the action works fine, but it doesn't modify the README, if you would like to check it out and see if there is a solution, it would be very helpful.

[caupolicanre]

Hey! just made a PR with this feature and another one of code block language customization.
If you'd like to see it, here is the Pull Request #491.