What key issues and themes does Oscar Wilde convey through his texts? Also, any particular literary devices he uses or how these themes relate to...

The previous answer beautifully and accurately analyzed
Wilde's texts in terms of his preferred writing styles and his predilection of genres.
My answer will therefore try to answer the same question but from the point of view of
Wilde's biography and how it blends in perfectly with specific works of his.  Most of
this information can be found in the superb biography written by Wilde's grandson,
Merlin Holland, The Scarlet Marquess and the Irish Peacock, which I
strongly recommend.

There is, indeed, a lot of personal
value at stake in specific texts by Wilde, particularly when you look at
:

• The Portrait of Mr. W.H. (story published in
  Blackwood Magazine in 1889)


• 
  The
  Picture of Dorian Gray (first a story, then a novel,
  1890)


• 
  The Importance of Being
  Earnest (play, St. James's Theater,
  1895)


• 
  An Ideal
  Husband (play, also
  1895)


• 
  Lady Windermere's
  fan (play, 1892)

The works produced
six years prior to Wilde's conviction, aside from being his most popular works, are also
works that feature the same conflicting issues that were going on in Wilde's life. This
time period also coincides with the chaotic relationship between Wilde and Alfred
Douglas. It is the latter's father, the Marquis of Queensberry, who would cause Wilde's
conviction. 

This being said, all of these works feature
major salient themes that were also preoccupying
Wilde:

• blackmail


• excess


• corruption


• homoeroticism


• criticism
  of women


• fear of
  consequences

Blackmail

By
1895, Wilde had been blackmailed by Charles Parker and
Alfred Wood (and others) over some sensually-suggestive letters that Wilde had sent to
Lord Alfred Douglas, his off and on love interest.Under the Criminal Law Amendment Act,
homosexual relations were considered illegal and punishable with a maximum sentence of
two years of hard labor. That is exactly what Wilde was accused off eventually in 1895,
and that was the sentence that he
served.

In Dorian Gray and
An Ideal Husband, and Lady Windermere  the
topic of blackmail is mentioned and talked about in ways that reflected Wilde's own
knowledge about these things. 

Excess and
consequences

During this time it is argued that Wilde was
also acting completely out of character by excessively
consuming food and drink. By the time of his trial, he was overweight and many say that
he was drinking uncontrollably. He was also spending excessively on his (male) love
interests and on page boys.

All of this got him in a huge
pickle with creditors (much like Ernest Worthing did in The Importance of
being Earnest). He eventually lost everything he owned when his assets were
seized by the government after his imprisonment.

The
characters of Jack (Ernest), his friend Algernon Moncrief, Dorian Gray, Lord Henry
Wotton, Lord Goring and Lord Darlington are all excessive
individuals who are always chased by the consequences of
their actions. All the 90-95 works also show Wilde's high preoccupation with the topic
of
"consequences".

Corruption

Prior
to his conviction, Wilde had been accused of corrupting
young men. In The Picture of Dorian Gray, the character of Lord Henry
is Wilde's mouthpiece, spreading Wilde's own ideas about the "modern hedonism" that
corrupts Dorian into horrid acts that reflect in his painted image. Similarly, the
dandies Moncrieff, Darlington and Wotton clearly are amoral therefore they have either
already corrupted someone or will corrupt someone
else. 

Homoeroticism and criticism of
women

Homoeroticism and criticism of
women permeates each and every one of these 90-95 works. "The Portrait of
Mr. W.H." talks about Shakespeare's love fixation with a young unknown male whose
initials are "W.H." 

Dorian Gray talks
about the main character reading a "certain book" that influences him in a way that he
starts to corrupt all young men who deal with him (Wilde was surreptitiously referring
to Huyssman's A Rebour which is a homoerotic
novel).

Women are consistently criticized or mocked. Lady
Bracknell is a snob, Cecily and Gwendolen are conniving airheads, Mrs. Cheveley is a con
artist, Sybil Vane is a weakling, Lady Wotton is "unkempt", and in general, men prefer
to hang out close together without the annoying presence of their female
counterparts.

Even in The Importance of Being
Earnest the male characters do a lot of naughty deceiving before they finally
fall in love with their respective female love
interests. 

Therefore, shortly before Wilde was arrested
and accused, he had already put a lot of his own life onto his works, presumably because
he was aware that something was going to happen eventually. Others say it is because he
had a penchant for self-sabotage and wanted to give himself away for once and for all.
Regardless the cause, it is certainly something to consider given the consistent way in
which he presents these themes over and over.