We don't follow you. Please rephrase your question.

Just find the classes by pressing F12 on the browser and opening Chrome or Firefox Console, it'll help you easily write CSS rules you need to customize.

Don't forget to delete the caches once you make changes.

This website should help you to write CSS rules: https://www.w3schools.com/cssref/default.asp

I will try to look into the theme files. Thank you.

I found the topic mentioned on this page as well but the solutions I found didnt help me either. Do you have any suggestions?

Try this CSS code:

#wpforo #wpforo-wrap .wpf-thread-head a {font-size: ***15px*** !important;}

The red- marked value can be changed as you like. 

Is there a way to change this? I have been looking through all the settings possibilities. I also couldnt find this exact topic on this site. Can it be done?

You should customize the wpForo theme files.

Please read attentively this doc: https://wpforo.com/docs/wpforo-v2/forum-themes/theme-files/ You'll find more information about wpForo theme files here

For an update-safe way of customization, please refer to this documentation: https://wpforo.com/docs/wpforo-v2/forum-themes/theme-customization/

I have attached a screenshot showing the two things I am trying to fix. The two things I am trying to edit is:

1) Change the topic font size

Ive tried this from Colors and Styles menu, but changing the Topics font size changes nothing (as opposed to the other two options, that works just fine). I also tried adding code in the Custom CSS-box but still nothing happened. I added the code from this page: Colors & Styles – wpForo – WordPress Forum Plugin

I found the topic mentioned on this page as well but the solutions I found didnt help me either. Do you have any suggestions?

2) Change the Topics icon

Is there a way to change this? I have been looking through all the settings possibilities. I also couldnt find this exact topic on this site. Can it be done?