Before you ask the obvious, let me preface this post by answering with “Why not??
Tableau preferences can be used to create custom colour palettes, changing defaults, etc. It is useful that there are several “preferences.tps” files. One in the \Documents\My Tableau Repository folder, and then one for each of the workbooks generated. Or, rather, the workbooks themselves contain a section for preferences.

Here, I am changing some of the colours in the search box in the map. Credit for the background research goes to dataplusscience.

Change the colour of the search box:

1. Make sure the workbook is not open in Tableau.
2. Work with the TWB file (if it is TWBX, just unzip it first and work with the unzipped TWB).
3. Open it with your preferred text editor.
4. Add the desired preferences at the beginning of the workbook (within the preferences section).
5. Save the file.
6. Reopen the workbook in Tableau.

What I find neat about this way of doing it (instead of modifying the preferences.tps file) is that these preferences travel with the workbook and are not fixed for the program. How not to to tailor your search box to your viz now?

These are the preferences I used. Find more at the dataplusscience post or at github post. Feel free to try new ones: worst case scenario just open the twb file again and delete them, no harm done!