Use theme provided by theme.js file and not css.

[njakuschona]

In the [theme.js] every used color should be written. (Primary, Seccondary, warning, success, error, ...)
These colors should be used throughout the UI and imported from the Theme instead of setting them in the css files.
For example in the check.css

success {
    color: #008643;
}

The theme of the color should be used instead.

How To:

1. import the Theme to the check.js file
   import { useTheme } from '@material-ui/core/styles';
2. use the Theme

function Status(status) {
    const theme = useTheme();
    switch (status.status) {
        case 'success':
            return <span style={{"color": theme.palette.success.main}}>Success</span>

[njakuschona]

belongs to #191

[nuest]

See also first changes in #199

[nuest]

It's enough for now that for our OJS use case that the use can set the important theme values manually.