Is your favorite Manga Reading/Streaming site is down or maintenance?  fret not just download Tachiyomi.

Tachiyomi is an free Open-Sourced application for reading Manga, Manhua, Manhwa, Webtoon and even comics from various websites it has many features such as:
•Dark Mode - Darken's the theme of the App

•Source Migration - Migrate downloaded manga and read/unread chapter progress from one source to another(ex. Migrating from Mangarock to MangaDex)

•Creating Backup's - Create Backup of the Library of saved manga

•Tracking - It will Sync Read Chapters to MyAnimelist, Anilist, Kitsu, Shikimori and Bangumi to record your progress (It is optional)

•Creating Custom Categories - Sort your library by making various categories you can make it by site(Mangarock, KissManga, MangaDex) or you can make it by Genre (Action,Fantasy,Romance) or by Manga Type (Manga,Manhwa,Manhua,Webtoon,Comics)

•Reader Settings - Here you can change the viewer settings from Default/Left to Right, Right to Left, Vertical, and Webtoon View(read by scrolling down)

To download click the link and select [Stable]: 
https://tachiyomi.org/download/