You can add these CSS blocks to your Thesis -> Custom CSS screen

CSS

1. Change menu text & hover text colors

.thesis-nav-menu a, .thesis-nav-menu li li a {color: #ccc}

.thesis-nav-menu li a:hover, .thesis-nav-menu .current-menu-item > a, .thesis-nav-menu li li a:hover {color: #000;}

2. Change previous / next button colors on blog

.next_posts a, .previous_posts a, #commentform #submit {
    background: #000;
    color: #FFF;
}

/* Hover Colors */
.next_posts a:hover,
.previous_posts a:hover {
    background-color: #CCC;
    border-color: #DDD;
    color: #FFF;
}

3. Reduce grey space at top of site

.wrapper {
    padding-top: 20px;
}

4. Reduce site header white space at top

.site-header .wrap {padding-top: 1rem;}