You can change the color entirely with CSS overrides. No need for a custom template. See the below on how to add CSS overrides to CB. You can also use Joomla template user.css file as well.
www.joomlapolis.com/blog/kyle/18711-template-css-overrides-made-easy
Below is an example of how to change the color of the button.
Code:
.cb_template .activityButtonMore {
color: #fff;
background-color: #17a2b8;
border-color: #17a2b8;
}
.cb_template .activityButtonMore:hover {
color: #fff;
background-color: #138496;
border-color: #117a8b;
}
.cb_template .activityButtonMore:not(:disabled):not(.disabled):active,
.cb_template .activityButtonMore:not(:disabled):not(.disabled).active {
color: #fff;
background-color: #117a8b;
border-color: #10707f;
}
.cb_template .activityButtonMore:focus,
.cb_template .activityButtonMore.focus {
color: #fff;
background-color: #138496;
border-color: #117a8b;
box-shadow: 0 0 0 0.2rem rgba(58, 176, 195, 0.5);
}
I've provided all the button states in the CSS, but you may only need to adjust some of that. So feel free to discard whatever isn't necessary for the look you're wanting.
Another option is to set CB to match whatever your Joomla template Bootstrap colors are. This is done by putting it in Bootstrap 5 Compatibility within CB > Configuration > Integrations. This should cause all button styling to match your template.
If you'd still like to make a custom template for CB Activity I recommend reading through the below topics as they provide examples of how to override specific template files.
www.joomlapolis.com/forum/single-add-on-support/247792-add-label-to-article-comments#342592
www.joomlapolis.com/forum/professional-members-support/247644-article-custom-fields#341862
The file you're needing to override will be /activity/rows.php as it contains the more button.