Please login wp-admin, go to Theone / General Settings / Custom Css
Paste the style code to Custom Css:

How can I change the colour of the dark grey background that appears behind the page when I hover over the sidebar?

body,#wrap{ background-color:#333; }

…I change the colour of the background that appears when you hover over a thumbnail and it spins (currently white text on dark grey)?

The background color is based on this post’s feature color. If you want to change default(dark grey), try to use cusotom css:

.flip_wrap_back_bg.post-bgcolor-default {
    background-color: #333;
}

This custom css is for font size

.flip_wrap_back_con h2 {
    font-size: 16px;
}