Customize WordPress CSS on top of an existing Theme

I have installed a theme in my WordPress website.

On top of the theme, I have added bbPress (forum)... unfortunately the CSS render very badly the forum features...

for example, the picture below show how the search button overlap with the search box

Search box-button overlapping

If I look at the css I can see this:

element.style {
}
vlog-bbpress.css?ver=1.8.1:9
#bbpress-forums #bbp-search-form {
    position: relative;
    float: none;
}

bbpress.css?ver….5.14-6684:429
#bbpress-forums #bbp-search-form {
    clear: left;
}

what should I change in the css??? (I have installed Simple Custom CSS plugin (https://en-ca.wordpress.org/plugins/simple-custom-css/)so I should not need to do any custom css, but I shoul just be able to add the custom CSS)

in case anyone of you want to have a look at the website, you can find it here (not live yet, just playing around)

http://italiancrypto.it/forums/

many thanks all

Regards

#bbpress-forums #bbp-search-form .button {
    margin-top:0;
}
#bbpress-forums #bbp-search-form #bbp_search {
    height: 42px;
}

You may need to include !important tags, depending on where your CSS overrides are injected. But only use if necessary.

eg:

#bbpress-forums #bbp-search-form .button {
    margin-top:0 !important;
}
#bbpress-forums #bbp-search-form #bbp_search {
    height: 42px !important;
}