In your design settings, there is a setting to change the color of the primary search buttons, however, this currently only applies to the default search form.
The search bar widget buttons for Price, Beds, and Baths, as well as the For Sale / All Types button (if you are using the Advanced Search), do not follow the design settings and will require additional code if you'd like to update them.
You can use the following code:
.sidx-container .sidx-searchform .sidx-basic .sidx-field {
background: #hexcode !important;
}
If you'd like to also change the font color, include this code just below the background color:
color: #hexcode !important;
Remember, place this code in the custom CSS box under Settings > Design. Clear your caches and check your web page in an incognito browser to see the change!