Well text origins from lang file English_front (or any other language).
It is also a shortcode under ....forum/shortcodes/catch/post_shortcodes (LAN_FORUM_2005) + view_shortcodes
Also in Forum icons template
LAN_FORUM_2005 ((that is the one you have to work around) is a define which represents the textual part you see.
Where it is used , you have to look around the surrounding code..
Styling of button would be something like(css bootstrap) btn btn-primary.. (the overall. button/box/img)
Since it also may depends on the theme you use, it can be a little nifty to apply any change.
As it looks like you only would like to change 1 related thing, it might be more difficult...
I would advice you to (any browser would be capable) RIGHT click ON the button, and select INSPECT (-or).
This should give you enough information normally what is used and which files..
Should you make any change backup before, and if you hardcode change the forum, use your own set, bundled (aka copy) the forum into your theme folder/temp...