Since update 4.2 of the Booking Calendar the calendar structure CSS file is located at the ../booking/css/calendar.css (please do not edit it or edit it very carefully). The calendar color skins are located in the ../booking/css/skins folder. Inside of these files (for better compatibility), please modify only following parameters:
* background , background-color, background-repeat, background-image, background-repeat, background-position
* border
* box-shadow, -moz-box-shadow, -webkit-box-shadow
* border-radius, -moz-border-radius, -webkit-border-radius
* color
* font-weight
* text-shadow
* text-transform

If you want to add new style.
Go to the folder “BOOKING_PLUGIN_PATH/css/skins/” and add new CSS file (the template you can get from this directory) to this directory. After this select this file at the general booking settings page and save settings.
If you want to customize exist skin.
Go to the folder “BOOKING_PLUGIN_PATH/css/skins/” open your file from this directory and customize it. After customization you need to select this style at the general booking settings page and save settings.

Important! Please note, if you will make upgrade of plugin to the new update, all plugin files is overwriting. Its means that your custom calendar skin also will be overwriting. That's why please backup your custom calendar skin before update and then restore it after update process.

Since new update 5.4 you do not need to backup/restore your custom calendar skins anymore. Just put your custom calendar skin to the special folder. Please save your own custom calendar skin to the /wp-content/uploads/wpbc_skins/ folder. You need to create this folder /wpbc_skins/ inside of your /wp-content/uploads/ folder, because this folder has not created automatically. Its will save your custom calendar skin, if you will make upgrade of plugin to new plugin update.

You can check this instruction how to edit CSS file from WordPress menu.