You can change the theme of the tag clouds (tabs and accordion) according to these instructions. There are some pre-defined color sets and you can also customize the colors to match your theme's colors.

If you are an advanced user, you can also write your own CSS. In the shortcode documentation you find under "Parameters" a chapter called "Advanced Styling". Each shortcode gives you some options how to assign an ID or a class to the enclosing elements. For example:

[tag_groups_cloud div_class="widget_tag_cloud"]

Then you can then refer to it in the CSS as ".widget_tag_cloud" and start styling the various elements:

.widget_tag_cloud li {
  color: #CCC;

You will need some experience with CSS and you also need to know how to explore the structure of the tag cloud through your browser's development tools.

If you just need to add a character before, after or between tags, you can achieve it without CSS by using one of the parameters "separator", "prepend", "append" in the shortcode.