CSS Modifications and Adjustments
It is pretty common that some CSS modifications are needed before going live with the WordPress website. Most of the times the default colours need to be modified to fit the Theme style or the company concept. Other times some margins, alignments and padding should be adjusted.
To make these kinds of changes and apply your preferred colors on the plugin views, go to Global > Configuration > Views and Layout – Preferred Colors.
Editing the default CSS rules of the component is easy when using a browser inspector tool that gives you the exact line to modify. It's too easy and it's actually wrong because any update will overwrite the main CSS file for styling the new features. The solution in this case is to not install the update by keeping an old and unsupported version of the extension. At least the work done will not go lost. This is just wrong because there is a better way to reach the goal.
VikBooking has a custom CSS file that gets loaded at last in the source code. This CSS file was made exactly for custom modifications and it should be used for any sort of adjustment. No updated will ever touch this file so you will never lose your modifications.
You should move onto this file all the CSS rules that were modified so that they will overwrite the default styles. This file can be modified directly from the Configuration page of the back-end, under the 3rd tab Views and Layout you will find the button to edit the Custom CSS.