Editor/Projects theming tidy up

[sra405]

The editor uses design tokens to apply themes for light mode, dark mode and projects.

The themes are largely defined in the editor's index.scss and the project site editor.scss but some themes are also defined at the component level (see, for example, the editor's tabs.scss).

It may be better to define all themes in a central file, which in the editor's case would also allow the standalone web component to receive a default theme through the InternalStyles.scss.

Fixes and issues

• !important is being used in the editor's Buttons.scss to override a secondary border colour. Can this be avoided?
• Editor's Tabs.scss doesn't apply themes consistent with Button.scss (ie, the button themes are defined in Index.scss). Review the tabs approach and make it consistent with the button
• Editor.scss uses placeholder colours forrpf-files-list-item-active and rpf-files-list-item-hover: check with design what the correct colours should be
• Dark mode tertiary button icons should be white