[Client] CSS 적용

> ### ISSUE
> * Group: `client`
> * Type: `feature`
> * Detail: fix actions from client redux
>
> ### TODO
> 1. [ ] mainpage - css적용
> 2. [ ] login - css적용
> 3. [ ] signup - css적용
> 4. [ ] board - css적용
> 5. [ ] mypage - css적용
>
> ### Estimated time
> > Pick one
>
> ### `3h`
> ### Labels
> * Estimated time: `E: 3h`
> * Group : `client`
> * Sprint: `bare-minimum`
> * Urgency: `Low`

Start by locating the client entry points and existing styles for the main page, login, signup, board, and mypage. Review how client Redux actions are currently wired before assessing the requested CSS work. Done means each of the five listed pages has its intended CSS applied, but the issue does not name specific files, tests, or visual requirements.

Written by the indexing model from the issue text.