How to change the colors of labels in booking form?

This customization is only for the Booking Calendar Free version. In the paid versions, you can easily configure booking form fields and their style at the advanced form configuration at the Booking > Settings > Fields page.

1) Please open this file ../wp-content/plugins/{Booking Calendar Folder}/css/client.css
and add to the bottom of that file this code:

.booking_form .control-label {
  color: #FFF;

Of course instead of white color #FFF you can set some other.

2) In the same way possible to change the color of text inside of the text fields, text-area elements and selectboxes. You need to add to the bottom of this file ../wp-content/plugins/{Booking Calendar Folder}/css/client.css this code:

.booking_form_div select,
.booking_form_div textarea,
.booking_form_div input[type="text"] {
  color: #FFF;

Again instead of white color #FFF you can set some other color.

Important! Please note after update of plugin to the newer version update, you will be need to make this fix again, because all Booking Calendar files will be overwritten.

How to edit calendar skin and define own colors in calendar?

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.