but everytime i change them, they go back to the original

Clear the forum and browser cache after customizing.

If you need more help, contact Pro Support and ask how much your changes will cost.

I have taken your advice and changed my forum to the 2022 theme and it looks awesome, I do want to change some colours but everytime i change them, they go back to the original, I am not very good with knowledge for coding, i want to hire someone to do a couple of changes for me, I will pay for the job.

I have added the new code and still not working,

The code above is for wpf-subscribe-topic only.
But why do you set this green in the color settings if you want black?

You won't lose any posts by changing the theme, but your design will logically look different.

Try it without

.wpf-button-outlined

#wpforo #wpforo-wrap .wpf-subscribe-topic {color: #000000 !important}

or check your color settings you have chosen this color ( Colors & Styles)

BTW. you are using the outdated classic theme, which no longer receives updates, you should switch to the 2022 theme and customize it to your needs

Try this code: 

#wpforo #wpforo-wrap .wpf-button-outlined.wpf-subscribe-topic {color: #000000 !important}

Thank you in advance.