To adjust the look of BwPostman to fit your template, basic knowledge of CSS and HTML is necessary. You also should use an extension in your browser to inspect the currently displayed page for check-up purpose. For Firefox, my preferred browser, there exists the developer tools. Such an extension also exists for Edge, Internet Explorer and in Chrome and Opera such a function is already implemented.
All elements, that are shown in the front-end, may be addressed individually at least since version 0.9.7 of BwPostman. The exact CSS path to the element you may find easily with above-mentioned extension. For all elements, BwPostman uses in front end, CSS Styles exists in the CSS file delivered with BwPostman. Most of them are empty and may be filled if needed.
For the component you find this file at
<Joomla-installation-directory>/components/com_bwpostman/assets/css/bwpostman.css
For the subscription module it is the file:
<Joomla-installation-directory>/modules/mod_bwpostman/css/bwpostman.css
For the overview module it is the file:
<Joomla-installation-directory>/modules/mod_bwpostman_overview/css/bwpostman_overview.css
You might change the CSS Styles in this files. But that only would reach until the next update of BwPostman, because while updating the file is set back to initial settings.
Another possibility is to set the desired CSS Styles in a CSS file of your template. But alike before there is still the problem, that the changes only last until the next update of the template.
Because of that I implemented a solution on a permanent basis since version 0.9.7 of BwPostman:
Create a file named com_bwpostman.css for the component and one file called mod_bwpostman.css for the module in the CSS directory of the template you use, or copy the delivered ones there. Or better: Copy the delivered CSS files to this location. These files will be read from BwPostman after all other CSS files. This means, that the CSS styles set in these files, override all previous read CSS styles.
Thus the path to these CSS files is:
<Joomla-installation-directory>/templates/<your template>/css/
With version 3.1.0 of BwPostman the CSS files are loaded at the templates instead of the view. This has the advantage, that You are able to use different CSS files including different paths with Your overrides.