Upgrade to Contact Form Pro if you want to change the button color (and other form styles) easily with the WordPress admin dashboard.
Please complete the following steps:
1) Go to the plugin settings page and open "Appearance" tab;
2) Find and enable the "Style options" checkbox.
3) Find the "Submit button" section and make necessary changes in the "Button color" field.
4) Save changes.
This article related to Contact Form by BestWebSoft plugin
Additional resources: Contact Form - User Guide
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