Change Post Comment and Search Button colour from black

you could add the necessary css to the bottom of your style sheet, but the changes will be overwritten if you update the theme.
I suggest making a child theme
https://codex.wordpress.org/Child_Themes as best way to go. A second way is to use a plugin
https://wordpress.org/plugins/simple-custom-css/
or Jetpack
info here
http://jetpack.me/support/custom-css/
In Firefox browser learn to use Firebug (just Google it) or Chrome has good dev tools (hit F12)
The code to add to stylesheet

.ttfmake-button, button, input[type="button"], input[type="reset"], input[type="submit"] {
    background: blue none repeat scroll 0 0;
}

or what ever colour you want.
When I view the page your search button is light grey. However the css to change the colour here is

.search-field {
    background-color: blue !important;
}

The ‘!important’ may not be necessary in child theme.
In Firefox browser a handy colour-picker addon is ColorZilla

menu-toggle generally is the class used to style the menu for mobile screen sizes, so that would be right. The id ‘menu-nav’ would style the desktop version of your menu.

.ttfmake-button, button, input[type="button"], input[type="reset"], input[type="submit"] {
    background: blue none repeat scroll 0 0;
}

works for me to style both “Post Comment and Search Button colour”.

If you try it again and it doesn’t work, could you copy your stylesheet (with the changes you have made) to http://pastebin.com/ so I can have a look.

when you put code in this forum remember to put code in between backticks as per http://codex.wordpress.org/Forum_Welcome